Printhead controller for nozzle fault correction

ABSTRACT

A printhead controller is provided for supplying print data to a printhead having first and second rows of inkjet nozzles configured to print ink of a similar color. The controller is configured to supply the print data to the printhead such that, in the event a nozzle in the first row is faulty, a corresponding nozzle in the second row prints at a position on print media at or adjacent a position where the faulty nozzle would otherwise have printed.

FIELD OF THE INVENTION

The present invention relates to the field of printer controllers, which receive print data (usually from an external source such as a network or personal computer) and provide it to one or more printheads or other printing mechanisms.

The invention has primarily been developed for use in a pagewidth inkjet printer in which considerable data processing and ordering is required of the printer controller, and will be described with reference to this example. However, it will be appreciated that the invention is not limited to any particular type of printing technology, and may be used in, for example, non-pagewidth and non-inkjet printing applications.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application:

7,374,266 7,427,117 7,448,707 7,281,330 10/854,503 7,328,956 10/854,509 7,188,928 7,093,989 7,377,609 10/854,495 10/854,498 10/854,511 7,390,071 10/854,525 10/854,526 10/854,516 7,252,353 7,267,417 10/854,505 10/854,493 7,275,805 7,314,261 10/854,490 7,281,777 7,290,852 7,484,831 10/854,523 10/854,527 10/854,524 10/854,520 10/854,514 10/854,519 10/854,513 10/854,499 10/854,501 7,266,661 7,243,193 10/854,518  

The disclosures of these co-pending applications are incorporated herein by cross-reference.

CROSS-REFERENCES

Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention. The disclosures of all of these co-pending applications are incorporated herein by cross reference.

7,249,108 6,566,858 6,331,946 6,246,970 6,442,525 7,346,586 09/505,951 6,374,354 7,246,098 6,816,968 6,757,832 6,334,190 6,745,331 7,249,109 7,509,292 10/636,283 7,416,280 7,252,366 7,488,051 7,360,865 10/727,181 10/727,162 7,377,608 7,399,043 7,121,639 7,165,824 7,152,942 10/727,157 7,181,572 7,096,137 7,302,592 7,278,034 7,188,282 10/727,159 10/727,180 10/727,179 10/727,192 10/727,274 10/727,164 7,523,111 10/727,198 10/727,158 10/754,536 10/754,938 10/727,160 6,795,215 6,859,289 6,977,751 6,398,332 6,394,573 6,622,923 6,747,760 6,921,144 7,454,617 7,194,629 10/791,792 7,182,267 7,025,279 6,857,571 6,817,539 6,830,198 6,992,791 7,038,809 6,980,323 7,148,992 7,139,091 6,947,173  

BACKGROUND

In a printhead module comprising a plurality of nozzles, there is always the possibility that a manufacturing defect, or over time in service, will cause one or more nozzle to fail. A failed nozzle can sometimes be corrected by error diffusion or color replacement. However, these solutions at best provide approximations of the color missing due to the defective nozzle.

The chances of a nozzle defect increases at least linearly with the number of nozzles on the printhead module, both through the increase in sample space for a failure to occur, and the reduction in nozzle size which requires higher tolerances. Defective chips reduce yield, which increases the effective cost of the remaining chips. Nozzles that fail in chips in service increase the costs of providing warranty cover.

The Applicant has designed a printhead that incorporates one or more redundant rows of nozzles. It would be desirable to provide a printer controller capable of providing data to such a printhead.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a printhead controller for supplying print data to a printhead having first and second rows of inkjet nozzles configured to print ink of a similar color, the controller being configured to supply the print data to the printhead such that, in the event a nozzle in the first row is faulty, a corresponding nozzle in the second row prints at a position on print media at or adjacent a position where the faulty nozzle would otherwise have printed.

Optionally, each nozzle in the first row is paired with a nozzle in the second row, such that each pair of nozzles is aligned in an intended direction of print media travel relative to the printhead.

Optionally, the printhead has a plurality of sets of the first and second rows.

Optionally, each of the sets of the first and second rows is configured to print in a single ink color.

Optionally, each of the rows includes an odd and an even sub-row, the odd and even sub-rows being offset with respect to each other in a direction of print media travel relative to the printhead.

Optionally, the odd and even sub-rows are transversely offset with respect to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Single SoPEC A4 Simplex

FIG. 2. Dual SoPEC A4 Simplex system

FIG. 3. Dual SoPEC A4 Duplex system

FIG. 4. Dual SoPEC A3 simplex system

FIG. 5. Quad SoPEC A3 duplex system

FIG. 6. SoPEC A4 Simplex system with extra SoPEC used as DRAM storage

FIG. 7. SoPEC A4 Simplex system with network connection to Host PC

FIG. 8. Document data flow

FIG. 9. Pages containing different numbers of bands

FIG. 10. Contents of a page band

FIG. 11. Page data path from host to SoPEC

FIG. 12. Page structure

FIG. 13. SoPEC System Top Level partition

FIG. 14. Proposed SoPEC CPU memory map (not to scale)

FIG. 15. Possible USB Topologies for Multi-SoPEC systems

FIG. 16. CPU block diagram

FIG. 17. CPU bus transactions

FIG. 18. Printhead Nozzle Layout for conceptual 36 Nozzle AB single segment printhead

FIG. 19. Paper and printhead nozzles relationship (example with D₁=D₂=5)

FIG. 20. Dot line store logical representation

FIG. 21. Conceptual view of 2 adjacent printhead segments possible row alignment

FIG. 22. Conceptual view of 2 adjacent printhead segments row alignment (as seen by the LLU)

FIG. 23. High level data flow diagram of LLU in context

FIG. 24. Paper and printhead nozzles relationship (example with D₁=D₂=5)

FIG. 25. Conceptual view of vertically misaligned printhead segment rows (external)

FIG. 26. Conceptual view of vertically misaligned printhead segment rows (internal)

FIG. 27. Conceptual view of color dependent vertically misaligned printhead segment rows (internal)

FIG. 28. Conceptual horizontal misalignment between segments

FIG. 29. Relative positions of dot fired (example cases)

FIG. 30. Example left and right margins

FIG. 31. Dot data generated and transmitted order

FIG. 32. Dotline FIFO data structure in DRAM (LLU specification)

FIG. 33. LLU partition

FIG. 34. DIU interface

FIG. 35. Interface controller state diagram

FIG. 36. Address generator logic

FIG. 37. Write pointer state machine

FIG. 38. PHI to linking printhead connection (Single SoPEC)

FIG. 39. PHI to linking printhead connection (2 SoPECs)

FIG. 40. CPU command word format

FIG. 41. Example data and command sequence on a print head channel

FIG. 42. PHI block partition

FIG. 43. Data generator state diagram

FIG. 44. PHI mode Controller

FIG. 45. Encoder RTL diagram

FIG. 46. 28-bit scrambler

FIG. 47. Printing with 1 SoPEC

FIG. 48. Printing with 2 SoPECs (existing hardware)

FIG. 49. Each SoPEC generates dot data and writes directly to a single printhead

FIG. 50. Each SoPEC generates dot data and writes directly to a single printhead

FIG. 51. Two SoPECs generate dots and transmit directly to the larger printhead

FIG. 52. Serial Load

FIG. 53. Parallel Load

FIG. 54. Two SoPECs generate dot data but only one transmits directly to the larger printhead

FIG. 55. Odd and Even nozzles on same shift register

FIG. 56. Odd and Even nozzles on different shift registers

FIG. 57. Interwoven shift registers

FIG. 58. Linking Printhead Concept

FIG. 59. Linking Printhead 30 ppm

FIG. 60. Linking Printhead 60 ppm

FIG. 61. Theoretical 2 tiles assembled as A-chip/A-chip—right angle join

FIG. 62. Two tiles assembled as A-chip/A-chip

FIG. 63. Magnification of color n in A-chip/A-chip

FIG. 64. A-chip/A-chip growing offset

FIG. 65. A-chip/A-chip aligned nozzles, sloped chip placement

FIG. 66. Placing multiple segments together

FIG. 67. Detail of a single segment in a multi-segment configuration

FIG. 68. Magnification of inter-slope compensation

FIG. 69. A-chip/B-chip

FIG. 70. A-chip/B-chip multi-segment printhead

FIG. 71. Two A-B-chips linked together

FIG. 72. Two A-B-chips with on-chip compensation

FIG. 73. Print construction and Nozzle position

FIG. 74. Conceptual horizontal misplacement between segments

FIG. 75. Printhead row positioning and default row firing order

FIG. 76. Firing order of fractionally misaligned segment

FIG. 77. Example of yaw in printhead IC misplacement

FIG. 78. Vertical nozzle spacing

FIG. 79. Single printhead chip plus connection to second chip

FIG. 80. Two printheads connected to form a larger printhead

FIG. 81. Colour arrangement.

FIG. 82. Nozzle Offset at Linking Ends

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Various aspects of the preferred and other embodiments will now be described.

Also throughout this description, “printhead module” and “printhead” are used somewhat interchangeably. Technically, a “printhead” comprises one or more “printhead modules”, but occasionally the former is used to refer to the latter. It should be clear from the context which meaning should be allocated to any use of the word “printhead”.

A SoPEC ASIC (Small office home office Print Engine Controller) suitable for use in price sensitive SoHo printer products is described. The SoPEC ASIC is intended to be a relatively low cost solution for linking printhead control, replacing the multichip solutions in larger more professional systems with a single chip. The increased cost competitiveness is achieved by integrating several systems such as a modified PEC1 printing pipeline, CPU control system, peripherals and memory sub-system onto one SoC ASIC, reducing component count and simplifying board design. SoPEC contains features making it suitable for multifunction or “all-in-one” devices as well as dedicated printing systems.

Basic features of the preferred embodiment of SoPEC include:

-   -   Continuous 30 ppm operation for 1600 dpi output at A4/Letter.     -   Linearly scalable (multiple SoPECs) for increased print speed         and/or page width.     -   192 MHz internal system clock derived from low-speed crystal         input     -   PEP processing pipeline, supports up to 6 color channels at 1         dot per channel per clock cycle     -   Hardware color plane decompression, tag rendering, halftoning         and compositing     -   Data formatting for Linking Printhead     -   Flexible compensation for dead nozzles, printhead misalignment         etc.     -   Integrated 20 Mbit (2.5 MByte) DRAM for print data and CPU         program store     -   LEON SPARC v8 32-bit RISC CPU     -   Supervisor and user modes to support multi-threaded software and         security     -   1 kB each of I-cache and D-cache, both direct mapped, with         optimized 256-bit fast cache update.     -   1× USB2.0 device port and 3× USB2.0 host ports (including         integrated PHYs)     -   Support high speed (480 Mbit/sec) and full speed (12 Mbit/sec)         modes of USB2.0     -   Provide interface to host PC, other SoPECs, and external devices         e.g. digital camera     -   Enable alternative host PC interfaces e.g. via external         USB/ethernet bridge     -   Glueless high-speed serial LVDS interface to multiple Linking         Printhead chips     -   64 remappable GPIOs, selectable between combinations of         integrated system control components:     -   2× LSS interfaces for QA chip or serial EEPROM     -   LED drivers, sensor inputs, switch control outputs     -   Motor controllers for stepper and brushless DC motors     -   Microprogrammed multi-protocol media interface for scanner,         external RAM/Flash, etc.     -   112-bit unique ID plus 112-bit random number on each device,         combined for security protocol support     -   IBM Cu-11 0.13 micron CMOS process, 1.5V core supply, 3.3V IO.     -   208 pin Plastic Quad Flat Pack

The following terms are used throughout this specification:

-   CPU Refers to CPU core, caching system and MMU. -   Host A PC providing control and print data to a Memjet printer. -   ISCMaster In a multi-SoPEC system, the ISCMaster (Inter SoPEC     Communication Master) is the SoPEC device that initiates     communication with other SoPECs in the system. The ISCMaster     interfaces with the host. -   ISCSlave In a multi-SoPEC system, an ISCSlave is a SoPEC device that     responds to communication initiated by the ISCMaster. -   LEON Refers to the LEON CPU core. -   LineSyncMaster The LineSyncMaster device generates the line     synchronisation pulse that all SoPECs in the system must synchronise     their line outputs to. -   Linking Printhead Refers to a page-width printhead constructed from     multiple linking printhead ICs -   Linking Printhead IC A MEMS IC. Multiple ICs link together to form a     complete printhead. An A4/Letter page width printhead requires 11     printhead ICs. -   Multi-SoPEC Refers to SoPEC based print system with multiple SoPEC     devices -   Netpage Refers to page printed with tags (normally in infrared ink). -   PEC1 Refers to Print Engine Controller version 1, precursor to SoPEC     used to control printheads constructed from multiple angled     printhead segments. -   PrintMaster The PrintMaster device is responsible for coordinating     all aspects of the print operation. There may only be one     PrintMaster in a system. -   QA Chip Quality Assurance Chip -   Storage SoPEC A SoPEC used as a DRAM store and which does not print. -   Tag Refers to pattern which encodes information about its position     and orientation which allow it to be optically located and its data     contents read.

The following acronyms and abbreviations are used in this specification

-   CFU Contone FIFO53 Unit -   CPU Central Processing Unit -   DIU DRAM Interface Unit -   DNC Dead Nozzle Compensator -   DRAM Dynamic Random Access Memory -   DWU DotLine Writer Unit -   GPIO General Purpose Input Output -   HCU Halftoner Compositor Unit -   ICU Interrupt Controller Unit -   LDB Lossless Bi-level Decoder -   LLU Line Loader Unit -   LSS Low Speed Serial interface -   MEMS Micro Electro Mechanical System -   MMI Multiple Media Interface -   MMU Memory Management Unit -   PCU SoPEC Controller Unit -   PHI PrintHead Interface -   PHY USB multi-port Physical Interface -   PSS Power Save Storage Unit -   RDU Real-time Debug Unit -   ROM Read Only Memory -   SFU Spot FIFO Unit -   SMG4 Silverbrook Modified Group 4. -   SoPEC Small office home office Print Engine Controller -   SRAM Static Random Access Memory -   TE Tag Encoder -   TFU Tag FIFO Unit -   TIM Timers Unit -   UDU USB Device Unit -   UHU USB Host Unit -   USB Universal Serial Bus

The preferred embodiment linking printhead produces 1600 dpi bi-level dots. On low-diffusion paper, each ejected drop forms a 22.5 m diameter dot. Dots are easily produced in isolation, allowing dispersed-dot dithering to be exploited to its fullest. Since the preferred form of the linking printhead is pagewidth and operates with a constant paper velocity, color planes are printed in good registration, allowing dot-on-dot printing. Dot-on-dot printing minimizes ‘muddying’ of midtones caused by inter-color bleed.

A page layout may contain a mixture of images, graphics and text. Continuous-tone (contone) images and graphics are reproduced using a stochastic dispersed-dot dither. Unlike a clustered-dot (or amplitude-modulated) dither, a dispersed-dot (or frequency-modulated) dither reproduces high spatial frequencies (i.e. image detail) almost to the limits of the dot resolution, while simultaneously reproducing lower spatial frequencies to their full color depth, when spatially integrated by the eye. A stochastic dither matrix is carefully designed to be free of objectionable low-frequency patterns when tiled across the image. As such its size typically exceeds the minimum size required to support a particular number of intensity levels (e.g. 16 16 8 bits for 257 intensity levels).

Human contrast sensitivity peaks at a spatial frequency of about 3 cycles per degree of visual field and then falls off logarithmically, decreasing by a factor of 100 beyond about 40 cycles per degree and becoming immeasurable beyond 60 cycles per degree. At a normal viewing distance of 12 inches (about 300 mm), this translates roughly to 200-300 cycles per inch (cpi) on the printed page, or 400-600 samples per inch according to Nyquist's theorem. In practice, contone resolution above about 300 ppi is of limited utility outside special applications such as medical imaging. Offset printing of magazines, for example, uses contone resolutions in the range 150 to 300 ppi. Higher resolutions contribute slightly to color error through the dither.

Black text and graphics are reproduced directly using bi-level black dots, and are therefore not anti-aliased (i.e. low-pass filtered) before being printed. Text should therefore be supersampled beyond the perceptual limits discussed above, to produce smoother edges when spatially integrated by the eye. Text resolution up to about 1200 dpi continues to contribute to perceived text sharpness (assuming low-diffusion paper).

A Netpage printer, for example, may use a contone resolution of 267 ppi (i.e. 1600 dpĩ 6), and a black text and graphics resolution of 800 dpi. A high end office or departmental printer may use a contone resolution of 320 ppi (1600 dpi/5) and a black text and graphics resolution of 1600 dpi. Both formats are capable of exceeding the quality of commercial (offset) printing and photographic reproduction.

The SoPEC device can be used in several printer configurations and architectures.

In the general sense, every preferred embodiment SoPEC-based printer architecture will contain:

-   -   One or more SoPEC devices.     -   One or more linking printheads.     -   Two or more LSS busses.     -   Two or more QA chips.     -   Connection to host, directly via USB2.0 or indirectly.     -   Connections between SoPECs (when multiple SoPECs are used).

Some example printer configurations as outlined in Section 6.2. The various system components are outlined briefly in Section 6.1.

The SoPEC device contains several system on a chip (SoC) components, as well as the print engine pipeline control application specific logic.

The PEP reads compressed page store data from the embedded memory, optionally decompresses the data and formats it for sending to the printhead. The print engine pipeline functionality includes expanding the page image, dithering the contone layer, compositing the black layer over the contone layer, rendering of Netpage tags, compensation for dead nozzles in the printhead, and sending the resultant image to the linking printhead.

SoPEC contains an embedded CPU for general-purpose system configuration and management. The CPU performs page and band header processing, motor control and sensor monitoring (via the GPIO) and other system control functions. The CPU can perform buffer management or report buffer status to the host. The CPU can optionally run vendor application specific code for general print control such as paper ready monitoring and LED status update.

A 2.5 Mbyte embedded memory buffer is integrated onto the SoPEC device, of which approximately 2 Mbytes are available for compressed page store data. A compressed page is divided into one or more bands, with a number of bands stored in memory. As a band of the page is consumed by the PEP for printing a new band can be downloaded. The new band may be for the current page or the next page.

Using banding it is possible to begin printing a page before the complete compressed page is downloaded, but care must be taken to ensure that data is always available for printing or a buffer underrun may occur.

A Storage SoPEC acting as a memory buffer could be used to provide guaranteed data delivery.

The embedded single-port USB2.0 device controller can be used either for interface to the host PC, or for communication with another SoPEC as an ISCSlave. It accepts compressed page data and control commands from the host PC or ISCMaster SoPEC, and transfers the data to the embedded memory for printing or downstream distribution.

The embedded three-port USB2.0 host controller enables communication with other SoPEC devices as a ISCMaster, as well as interfacing with external chips (e.g. for Ethernet connection) and external USB devices, such as digital cameras.

SoPEC contains embedded controllers for a variety of printer system components such as motors, LEDs etc, which are controlled via SoPEC's GPIOs. This minimizes the need for circuits external to SoPEC to build a complete printer system.

The linking printhead is constructed by abutting a number of printhead ICs together. Each SoPEC can drive up to 12 printhead ICs at data rates up to 30 ppm or 6 printhead ICs at data rates up to 60 ppm. For higher data rates, or wider printheads, multiple SoPECs must be used.

Each SoPEC device has 2 LSS system buses for communication with QA devices for system authentication and ink usage accounting. The number of QA devices per bus and their position in the system is unrestricted with the exception that PRINTER_QA and INK_QA devices should be on separate LSS busses.

Each SoPEC system can have several QA devices. Normally each printing SoPEC will have an associated PRINTER_QA. Ink cartridges will contain an INK_QA chip. PRINTER_QA and INK_QA devices should be on separate LSS busses. All QA chips in the system are physically identical with flash memory contents defining PRINTER_QA from INK_QA chip.

In a multi-SoPEC system, the primary communication channel is from a USB2.0 Host port on one SoPEC (the ISCMaster), to the USB2.0 Device port of each of the other SoPECs (ISCSlaves). If there are more ISCSlave SoPECs than available USB Host ports on the ISCMaster, additional connections could be via a USB Hub chip, or daisy-chained SoPEC chips. Typically one or more of SoPEC's GPIO signals would also be used to communicate specific events between multiple SoPECs.

The communication between the host PC and the ISCMaster SoPEC may involve an external chip or subsystem, to provide a non-USB host interface, such as ethernet or WiFi. This subsystem may also contain memory to provide an additional buffered band/page store, which could provide guaranteed bandwidth data deliver to SoPEC during complex page prints.

In FIG. 1, a single SoPEC device is used to control a linking printhead with 11 printhead ICs. The SoPEC receives compressed data from the host through its USB device port. The compressed data is processed and transferred to the printhead. This arrangement is limited to a speed of 30 ppm. The single SoPEC also controls all printer components such as motors, LEDs, buttons etc, either directly or indirectly.

In FIG. 2, two SoPECs control a single linking printhead, to provide 60 ppm A4 printing. Each SoPEC drives 5 or 6 of the printheads ICs that make up the complete printhead. SoPEC #0 is the ISCMaster, SoPEC #1 is an ISCSlave. The ISCMaster receives all the compressed page data for both SoPECs and re-distributes the compressed data for the ISCSlave over a local USB bus. There is a total of 4 MBytes of page store memory available if required. Note that, if each page has 2 MBytes of compressed data, the USB2.0 interface to the host needs to run in high speed (not full speed) mode to sustain 60 ppm printing. (In practice, many compressed pages will be much smaller than 2 MBytes). The control of printer components such as motors, LEDs, buttons etc, is shared between the 2 SoPECs in this configuration.

In FIG. 3, two SoPEC devices are used to control two printheads. Each printhead prints to opposite sides of the same page to achieve duplex printing. SoPEC #0 is the ISCMaster, SoPEC #1 is an ISCSlave. The ISCMaster receives all the compressed page data for both SoPECs and re-distributes the compressed data for the ISCSlave over a local USB bus. This configuration could print 30 double-sided pages per minute. In FIG. 4, two SoPEC devices are used to control one A3 linking printhead, constructed from 16 printhead ICs. Each SoPEC controls 8 printhead ICs. This system operates in a similar manner to the 60 ppm A4 system in FIG. 2, although the speed is limited to 30 ppm at A3, since each SoPEC can only drive 6 printhead ICs at 60 ppm speeds. A total of 4 Mbyte of page store is available, this allows the system to use compression rates as in a single SoPEC A4 architecture, but with the increased page size of A3.

In FIG. 5 a four SoPEC system is shown. It contains 2 A3 linking printheads, one for each side of an A3 page. Each printhead contain 16 printhead ICs, each SoPEC controls 8 printhead ICs. SoPEC #0 is the ISCMaster with the other SoPECs as ISCSlaves. Note that all 3 USB Host ports on SoPEC #0 are used to communicate with the 3 ISCSlave SoPECs. In total, the system contains 8 Mbytes of compressed page store (2 Mbytes per SoPEC), so the increased page size does not degrade the system print quality, from that of an A4 simplex printer. The ISCMaster receives all the compressed page data for all SoPECs and re-distributes the compressed data over the local USB bus to the ISCSlaves. This configuration could print 30 double-sided A3 sheets per minute.

Extra SoPECs can be used for DRAM storage e.g. in FIG. 6 an A4 simplex printer can be built with a single extra SoPEC used for DRAM storage. The DRAM SoPEC can provide guaranteed bandwidth delivery of data to the printing SoPEC. SoPEC configurations can have multiple extra SoPECs used for DRAM storage.

FIG. 7 shows a configuration in which the connection from the host PC to the printer is an ethernet network, rather than USB. In this case, one of the USB Host ports on SoPEC interfaces to a external device that provide ethernet-to-USB bridging. Note that some networking software support in the bridging device might be required in this configuration. A Flash RAM will be required in such a system, to provide SoPEC with driver software for the Ethernet bridging function.

Because of the page-width nature of the linking printhead, each page must be printed at a constant speed to avoid creating visible artifacts. This means that the printing speed can't be varied to match the input data rate. Document rasterization and document printing are therefore decoupled to ensure the printhead has a constant supply of data. A page is never printed until it is fully rasterized. This can be achieved by storing a compressed version of each rasterized page image in memory.

This decoupling also allows the RIP(s) to run ahead of the printer when rasterizing simple pages, buying time to rasterize more complex pages.

Because contone color images are reproduced by stochastic dithering, but black text and line graphics are reproduced directly using dots, the compressed page image format contains a separate foreground bi-level black layer and background contone color layer. The black layer is composited over the contone layer after the contone layer is dithered (although the contone layer has an optional black component). A final layer of Netpage tags (in infrared, yellow or black ink) is optionally added to the page for printout.

FIG. 8 shows the flow of a document from computer system to printed page.

At 267 ppi for example, an A4 page (8.26 inches 11.7 inches) of contone CMYK data has a size of 26.3 MB. At 320 ppi, an A4 page of contone data has a size of 37.8 MB. Using lossy contone compression algorithms such as JPEG, contone images compress with a ratio up to 10:1 without noticeable loss of quality, giving compressed page sizes of 2.63 MB at 267 ppi and 3.78 MB at 320 ppi.

At 800 dpi, an A4 page of bi-level data has a size of 7.4 MB. At 1600 dpi, a Letter page of bi-level data has a size of 29.5 MB. Coherent data such as text compresses very well. Using lossless bi-level compression algorithms such as SMG4 fax as discussed in Section 8.1.2.3.1, ten-point plain text compresses with a ratio of about 50:1. Lossless bi-level compression across an average page is about 20:1 with 10:1 possible for pages which compress poorly. The requirement for SoPEC is to be able to print text at 10:1 compression. Assuming 10:1 compression gives compressed page sizes of 0.74 MB at 800 dpi, and 2.95 MB at 1600 dpi.

Once dithered, a page of CMYK contone image data consists of 116 MB of bi-level data. Using lossless bi-level compression algorithms on this data is pointless precisely because the optimal dither is stochastic—i.e. since it introduces hard-to-compress disorder.

Netpage tag data is optionally supplied with the page image. Rather than storing a compressed bi-level data layer for the Netpage tags, the tag data is stored in its raw form. Each tag is supplied up to 120 bits of raw variable data (combined with up to 56 bits of raw fixed data) and covers up to a 6 mm 6 mm area (at 1600 dpi). The absolute maximum number of tags on a A4 page is 15,540 when the tag is only 2 mm 2 mm (each tag is 126 dots 126 dots, for a total coverage of 148 tags 105 tags). 15,540 tags of 128 bits per tag gives a compressed tag page size of 0.24 MB.

The multi-layer compressed page image format therefore exploits the relative strengths of lossy JPEG contone image compression, lossless bi-level text compression, and tag encoding. The format is compact enough to be storage-efficient, and simple enough to allow straightforward real-time expansion during printing.

Since text and images normally don't overlap, the normal worst-case page image size is image only, while the normal best-case page image size is text only. The addition of worst case Netpage tags adds 0.24 MB to the page image size. The worst-case page image size is text over image plus tags. The average page size assumes a quarter of an average page contains images.

The Host PC rasterizes and compresses the incoming document on a page by page basis. The page is restructured into bands with one or more bands used to construct a page. The compressed data is then transferred to the SoPEC device directly via a USB link, or via an external bridge e.g. from ethernet to USB. A complete band is stored in SoPEC embedded memory. Once the band transfer is complete the SoPEC device reads the compressed data, expands the band, normalizes contone, bi-level and tag data to 1600 dpi and transfers the resultant calculated dots to the linking printhead.

The document data flow is

-   -   The RIP software rasterizes each page description and compress         the rasterized page image.     -   The infrared layer of the printed page optionally contains         encoded Netpage tags at a programmable density.     -   The compressed page image is transferred to the SoPEC device via         the USB (or ethernet), normally on a band by band basis.     -   The print engine takes the compressed page image and starts the         page expansion.     -   The first stage page expansion consists of 3 operations         performed in parallel     -   expansion of the JPEG-compressed contone layer     -   expansion of the SMG4 fax compressed bi-level layer     -   encoding and rendering of the bi-level tag data.     -   The second stage dithers the contone layer using a programmable         dither matrix, producing up to four bi-level layers at         full-resolution.     -   The third stage then composites the bi-level tag data layer, the         bi-level SMG4 fax de-compressed layer and up to four bi-level         JPEG de-compressed layers into the full-resolution page image.     -   A fixative layer is also generated as required.     -   The last stage formats and prints the bi-level data through the         linking printhead via the printhead interface.

The SoPEC device can print a full resolution page with 6 color planes. Each of the color planes can be generated from compressed data through any channel (either JPEG compressed, bi-level SMG4 fax compressed, tag data generated, or fixative channel created) with a maximum number of 6 data channels from page RIP to linking printhead color planes.

The mapping of data channels to color planes is programmable. This allows for multiple color planes in the printhead to map to the same data channel to provide for redundancy in the printhead to assist dead nozzle compensation.

Also a data channel could be used to gate data from another data channel. For example in stencil mode, data from the bilevel data channel at 1600 dpi can be used to filter the contone data channel at 320 dpi, giving the effect of 1600 dpi edged contone images, such as 1600 dpi color text.

The SoPEC device typically stores a complete page of document data on chip. The amount of storage available for compressed pages is limited to 2 Mbytes, imposing a fixed maximum on compressed page size. A comparison of the compressed image sizes indicates that SoPEC would not be capable of printing worst case pages unless they are split into bands and printing commences before all the bands for the page have been downloaded. The page sizes in the table are shown for comparison purposes and would be considered reasonable for a professional level printing system. The SoPEC device is aimed at the consumer level and would not be required to print pages of that complexity.

If a document with more complex pages is required, the page RIP software in the host PC can determine that there is insufficient memory storage in the SoPEC for that document. In such cases the RIP software can take two courses of action:

-   -   It can increase the compression ratio until the compressed page         size will fit in the SoPEC device, at the expense of print         quality, or     -   It can divide the page into bands and allow SoPEC to begin         printing a page band before all bands for that page are         downloaded.

Once SoPEC starts printing a page it cannot stop; if SoPEC consumes compressed data faster than the bands can be downloaded a buffer underrun error could occur causing the print to fail. A buffer underrun occurs if a line synchronisation pulse is received before a line of data has been transferred to the printhead.

Other options which can be considered if the page does not fit completely into the compressed page store are to slow the printing or to use multiple SoPECs to print parts of the page. Alternatively, a number of methods are available to provide additional local page data storage with guaranteed bandwidth to SoPEC, for example a Storage SoPEC.

The preceding sections have described the document flow for printing from a host PC in which the RIP on the host PC does much of the management work for SoPEC. SoPEC also supports printing of images directly from other sources, such as a digital camera or scanner, without the intervention of a host PC.

In such cases, SoPEC receives image data (and associated metadata) into its DRAM via a USB host or other local media interface. Software running on SoPEC's CPU determines the image format (e.g. compressed or non-compressed, RGB or CMY, etc.), and optionally applies image processing algorithms such as color space conversion. The CPU then makes the data to be printed available to the PEP pipeline. SoPEC allows various PEP pipeline stages to be bypassed, for example JPEG decompression. Depending on the format of the data to be printed, PEP hardware modules interact directly with the CPU to manage DRAM buffers, to allow streaming of data from an image source (e.g. scanner) to the printhead interface without overflowing the limited on-chip DRAM.

When rendering a page, the RIP produces a page header and a number of bands (a non-blank page requires at least one band) for a page. The page header contains high level rendering parameters, and each band contains compressed page data. The size of the band will depend on the memory available to the RIP, the speed of the RIP, and the amount of memory remaining in SoPEC while printing the previous band(s). FIG. 9 shows the high level data structure of a number of pages with different numbers of bands in the page.

Each compressed band contains a mandatory band header, an optional bi-level plane, optional sets of interleaved contone planes, and an optional tag data plane (for Netpage enabled applications). Since each of these planes is optional, the band header specifies which planes are included with the band. FIG. 10 gives a high-level breakdown of the contents of a page band.

A single SoPEC has maximum rendering restrictions as follows:

-   -   1 bi-level plane     -   1 contone interleaved plane set containing a maximum of 4         contone planes     -   1 tag data plane     -   a linking printhead with a maximum of 12 printhead ICs

The requirement for single-sided A4 single SoPEC printing at 30 ppm is

-   -   average contone JPEG compression ratio of 10:1, with a local         minimum compression ratio of 5:1 for a single line of         interleaved JPEG blocks.     -   average bi-level compression ratio of 10:1, with a local minimum         compression ratio of 1:1 for a single line.

If the page contains rendering parameters that exceed these specifications, then the RIP or the Host PC must split the page into a format that can be handled by a single SoPEC. In the general case, the SoPEC CPU must analyze the page and band headers and generate an appropriate set of register write commands to configure the units in SoPEC for that page. The various bands are passed to the destination SoPEC(s) to locations in DRAM determined by the host.

The host keeps a memory map for the DRAM, and ensures that as a band is passed to a SoPEC, it is stored in a suitable free area in DRAM. Each SoPEC receives its band data via its USB device interface. Band usage information from the individual SoPECs is passed back to the host. FIG. 11 shows an example data flow for a page destined to be printed by a single SoPEC.

SoPEC has an addressing mechanism that permits circular band memory allocation, thus facilitating easy memory management. However it is not strictly necessary that all bands be stored together. As long as the appropriate registers in SoPEC are set up for each band, and a given band is contiguous, the memory can be allocated in any way.

This section describes a possible format of compressed pages expected by the embedded CPU in SoPEC. The format is generated by software in the host PC and interpreted by embedded software in SoPEC. This section indicates the type of information in a page format structure, but implementations need not be limited to this format. The host PC can optionally perform the majority of the header processing.

The compressed format and the print engines are designed to allow real-time page expansion during printing, to ensure that printing is never interrupted in the middle of a page due to data underrun.

The page format described here is for a single black bi-level layer, a contone layer, and a Netpage tag layer. The black bi-level layer is defined to composite over the contone layer. The black bi-level layer consists of a bitmap containing a 1-bit opacity for each pixel. This black layer matte has a resolution which is an integer or non-integer factor of the printer's dot resolution. The highest supported resolution is 1600 dpi, i.e. the printer's full dot resolution.

The contone layer, optionally passed in as YCrCb, consists of a 24-bit CMY or 32-bit CMYK color for each pixel. This contone image has a resolution which is an integer or non-integer factor of the printer's dot resolution. The requirement for a single SoPEC is to support 1 side per 2 seconds A4/Letter printing at a resolution of 267 ppi, i.e. one-sixth the printer's dot resolution.

Non-integer scaling can be performed on both the contone and bi-level images. Only integer scaling can be performed on the tag data.

The black bi-level layer and the contone layer are both in compressed form for efficient storage in the printer's internal memory.

A single SoPEC is able to print with full edge bleed for A4/Letter paper using the linking printhead. It imposes no margins and so has a printable page area which corresponds to the size of its paper. The target page size is constrained by the printable page area, less the explicit (target) left and top margins specified in the page description. These relationships are illustrated below.

Apart from being implicitly defined in relation to the printable page area, each page description is complete and self-contained. There is no data stored separately from the page description to which the page description refers. The page description consists of a page header which describes the size and resolution of the page, followed by one or more page bands which describe the actual page content.

The page header contains a signature and version which allow the CPU to identify the page header format. If the signature and/or version are missing or incompatible with the CPU, then the CPU can reject the page.

The contone flags define how many contone layers are present, which typically is used for defining whether the contone layer is CMY or CMYK. Additionally, if the color planes are CMY, they can be optionally stored as YCrCb, and further optionally color space converted from CMY directly or via RGB. Finally the contone data is specified as being either JPEG compressed or non-compressed.

The page header defines the resolution and size of the target page. The bi-level and contone layers are clipped to the target page if necessary. This happens whenever the bi-level or contone scale factors are not factors of the target page width or height.

The target left, top, right and bottom margins define the positioning of the target page within the printable page area.

The tag parameters specify whether or not Netpage tags should be produced for this page and what orientation the tags should be produced at (landscape or portrait mode). The fixed tag data is also provided.

The contone, bi-level and tag layer parameters define the page size and the scale factors. The bi-level layer parameters define the height of the black band, and the size of its compressed band data. The variable-size black data follows the page band header. The contone layer parameters define the height of the contone band, and the size of its compressed page data. The variable-size contone data follows the black data. The tag band data is the set of variable tag data half-lines as required by the tag encoder. The tag band data follows the contone data.

The start of each variable-size segment of band data should be aligned to a 256-bit DRAM word boundary.

The following sections describe the format of the compressed bi-level layers and the compressed contone layer.

The (typically 1600 dpi) black bi-level layer is losslessly compressed using Silverbrook Modified Group 4 (SMG4) compression which is a version of Group 4 Facsimile compression without Huffman and with simplified run length encodings. Typically compression ratios exceed 10:1.

The Small Office Home Office Print Engine Controller (SoPEC) is a page rendering engine ASIC that takes compressed page images as input, and produces decompressed page images at up to 6 channels of bi-level dot data as output. The bi-level dot data is generated for the Memjet linking printhead. The dot generation process takes account of printhead construction, dead nozzles, and allows for fixative generation.

A single SoPEC can control up to 12 linking printheads and up to 6 color channels at >10,000 lines/sec, equating to 30 pages per minute. A single SoPEC can perform full-bleed printing of A4 and Letter pages. The 6 channels of colored ink are the expected maximum in a consumer SOHO, or office Memjet printing environment:

-   -   CMY, for regular color printing.     -   K, for black text, line graphics and gray-scale printing.     -   IR (infrared), for Netpage-enabled applications.     -   F (fixative), to enable printing at high speed. Because the         Memjet printer is capable of printing so fast, a fixative may be         required on specific media types (such as calendared paper) to         enable the ink to dry before the page touches a previously         printed page. Otherwise the pages may bleed on each other. In         low speed printing environments, and for plain and photo paper,         the fixative is not be required.

SoPEC is color space agnostic. Although it can accept contone data as CMYX or RGBX, where X is an optional 4th channel (such as black), it also can accept contone data in any print color space. Additionally, SoPEC provides a mechanism for arbitrary mapping of input channels to output channels, including combining dots for ink optimization, generation of channels based on any number of other channels etc. However, inputs are typically CMYK for contone input, K for the bi-level input, and the optional Netpage tag dots are typically rendered to an infra-red layer. A fixative channel is typically only generated for fast printing applications.

SoPEC is resolution agnostic. It merely provides a mapping between input resolutions and output resolutions by means of scale factors. The expected output resolution is 1600 dpi, but SoPEC actually has no knowledge of the physical resolution of the linking printhead.

SoPEC is page-length agnostic. Successive pages are typically split into bands and downloaded into the page store as each band of information is consumed and becomes free. SoPEC provides mechanisms for synchronization with other SoPECs. This allows simple multi-SoPEC solutions for simultaneous A3/A4/Letter duplex printing. However, SoPEC is also capable of printing only a portion of a page image. Combining synchronization functionality with partial page rendering allows multiple SoPECs to be readily combined for alternative printing requirements including simultaneous duplex printing and wide format printing.

The required printing rate for a single SoPEC is 30 sheets per minute with an inter-sheet spacing of 4 cm. To achieve a 30 sheets per minute print rate, this requires:

-   -   300 mm×63 (dot/mm)/2 sec=105.8 seconds per line, with no         inter-sheet gap.     -   340 mm×63 (dot/mm)/2 sec=93.3 seconds per line, with a 4 cm         inter-sheet gap.

A printline for an A4 page consists of 13824 nozzles across the page. At a system clock rate of 192 MHz, 13824 dots of data can be generated in 69.2 seconds. Therefore data can be generated fast enough to meet the printing speed requirement.

Once generated, the data must be transferred to the printhead. Data is transferred to the printhead ICs using a 288 MHz clock (3/2 times the system clock rate). SoPEC has 6 printhead interface ports running at this clock rate. Data is 8b/10b encoded, so the throughput per port is 0.8×288=230.4 Mb/sec. For 6 color planes, the total number of dots per printhead IC is 1280×6=7680, which takes 33.3 seconds to transfer. With 6 ports and 11 printhead ICs, 5 of the ports address 2 ICs sequentially, while one port addresses one IC and is idle otherwise. This means all data is transferred on 66.7 seconds (plus a slight overhead). Therefore one SoPEC can transfer data to the printhead fast enough for 30 ppm printing.

From the highest point of view the SoPEC device consists of 3 distinct subsystems

-   -   CPU Subsystem     -   DRAM Subsystem     -   Print Engine Pipeline (PEP) Subsystem

See FIG. 13 for a block level diagram of SoPEC.

The CPU subsystem controls and configures all aspects of the other subsystems. It provides general support for interfacing and synchronising the external printer with the internal print engine. It also controls the low speed communication to the QA chips. The CPU subsystem contains various peripherals to aid the CPU, such as GPIO (includes motor control), interrupt controller, LSS Master, MMI and general timers. The CPR block provides a mechanism for the CPU to powerdown and reset individual sections of SoPEC. The UDU and UHU provide high-speed USB2.0 interfaces to the host, other SoPEC devices, and other external devices. For security, the CPU supports user and supervisor mode operation, while the CPU subsystem contains some dedicated security components.

The DRAM subsystem accepts requests from the CPU, UHU, UDU, MMI and blocks within the PEP subsystem. The DRAM subsystem (in particular the DIU) arbitrates the various requests and determines which request should win access to the DRAM. The DIU arbitrates based on configured parameters, to allow sufficient access to DRAM for all requesters. The DIU also hides the implementation specifics of the DRAM such as page size, number of banks, refresh rates etc.

The Print Engine Pipeline (PEP) subsystem accepts compressed pages from DRAM and renders them to bi-level dots for a given print line destined for a printhead interface that communicates directly with up to 12 linking printhead ICs.

The first stage of the page expansion pipeline is the CDU, LBD and TE. The CDU expands the JPEG-compressed contone (typically CMYK) layer, the LBD expands the compressed bi-level layer (typically K), and the TE encodes Netpage tags for later rendering (typically in IR, Y or K ink). The output from the first stage is a set of buffers: the CFU, SFU, and TFU. The CFU and SFU buffers are implemented in DRAM.

The second stage is the HCU, which dithers the contone layer, and composites position tags and the bi-level spot0 layer over the resulting bi-level dithered layer. A number of options exist for the way in which compositing occurs. Up to 6 channels of bi-level data are produced from this stage. Note that not all 6 channels may be present on the printhead. For example, the printhead may be CMY only, with K pushed into the CMY channels and IR ignored. Alternatively, the position tags may be printed in K or Y if IR ink is not available (or for testing purposes).

The third stage (DNC) compensates for dead nozzles in the printhead by color redundancy and error diffusing dead nozzle data into surrounding dots.

The resultant bi-level 6 channel dot-data (typically CMYK-IRF) is buffered and written out to a set of line buffers stored in DRAM via the DWU.

Finally, the dot-data is loaded back from DRAM, and passed to the printhead interface via a dot FIFO. The dot FIFO accepts data from the LLU up to 2 dots per system clock cycle, while the PHI removes data from the FIFO and sends it to the printhead at a maximum rate of 1.5 dots per system clock cycle.

Looking at FIG. 13, the various units are described here in summary form:

TABLE 1 Units within SoPEC Unit Subsystem Acronym Unit Name Description DRAM DIU DRAM interface Provides the interface for DRAM unit read and write access for the various PEP units CPU, UDU, UHU, and MMI. The DIU provides arbitration between competing units controls DRAM access. DRAM Embedded DRAM 20 Mbits of embedded DRAM, CPU CPU Central Processing CPU for system configuration and Unit control MMU Memory Limits access to certain memory Management Unit address areas in CPU user mode RDU Real-time Debug Facilitates the observation of the Unit contents of most of the CPU addressable registers in SoPEC in addition to some pseudo-registers in realtime. TIM General Timer Contains watchdog and general system timers LSS Low Speed Serial Low level controller for interfacing Interfaces with the QA chips GPIO General Purpose General IO controller, with built-in IOs Motor control unit, LED pulse units and de-glitch circuitry MMI Multi-Media Generic Purpose Engine for protocol Interface generation and control with integrated DMA controller. ROM Boot ROM 16 KBytes of System Boot ROM code ICU Interrupt General Purpose interrupt controller Controller Unit with configurable priority, and masking. CPR Clock, Power and Central Unit for controlling and Reset block generating the system clocks and resets and powerdown mechanisms PSS Power Save Storage retained while system is Storage powered down USB Universal Serial USB multiport (4) physical interface. PHY Bus (USB) Physical UHU USB Host Unit USB host controller interface with integrated DIU DMA controller UDU USB Device Unit USB Device controller interface with integrated DIU DMA controller Print PCU PEP controller Provides external CPU with the Engine means to read and write PEP Unit Pipeline registers, and read and write DRAM (PEP) in single 32-bit chunks. CDU Contone decoder Expands JPEG compressed contone unit layer and writes decompressed contone to DRAM CFU Contone FIFO Unit Provides line buffering between CDU and HCU LBD Lossless Bi-level Expands compressed bi-level layer. Decoder SFU Spot FIFO Unit Provides line buffering between LBD and HCU TE Tag encoder Encodes tag data into line of tag dots. TFU Tag FIFO Unit Provides tag data storage between TE and HCU HCU Halftoner Dithers contone layer and compositor unit composites the bi-level spot 0 and position tag dots. DNC Dead Nozzle Compensates for dead nozzles by Compensator color redundancy and error diffusing dead nozzle data into surrounding dots. DWU Dotline Writer Writes out the 6 channels of dot data Unit for a given printline to the line store DRAM LLU Line Loader Unit Reads the expanded page image from line store, formatting the data appropriately for the linking printhead. PHI PrintHead Interface Is responsible for sending dot data to the linking printheads and for providing line synchronization between multiple SoPECs. Also provides test interface to printhead such as temperature monitoring and Dead Nozzle Identification.

SoPEC must address

-   -   20 Mbit DRAM.     -   PCU addressed registers in PEP.     -   CPU-subsystem addressed registers.

SoPEC has a unified address space with the CPU capable of addressing all CPU-subsystem and PCU-bus accessible registers (in PEP) and all locations in DRAM. The CPU generates byte-aligned addresses for the whole of SoPEC.

22 bits are sufficient to byte address the whole SoPEC address space.

The embedded DRAM is composed of 256-bit words. Since the CPU-subsystem may need to write individual bytes of DRAM, the DIU is byte addressable. 22 bits are required to byte address 20 Mbits of DRAM.

Most blocks read or write 256-bit words of DRAM. For these blocks only the top 17 bits i.e. bits 21 to 5 are required to address 256-bit word aligned locations.

The exceptions are

-   -   CDU which can write 64-bits so only the top 19 address bits i.e.         bits 21-3 are required.     -   The CPU-subsystem always generates a 22-bit byte-aligned DIU         address but it will send flags to the DIU indicating whether it         is an 8, 16 or 32-bit write.     -   The UHU and UDU generate 256-bit aligned addresses, with a         byte-wise write mask associated with each data word, to allow         effective byte addressing of the DRAM.

Regardless of the size no DIU access is allowed to span a 256-bit aligned DRAM word boundary.

PEP Unit configuration registers which specify DRAM locations should specify 256-bit aligned DRAM addresses i.e. using address bits 21:5. Legacy blocks from PEC1 e.g. the LBD and TE may need to specify 64-bit aligned DRAM addresses if these reused blocks DRAM addressing is difficult to modify. These 64-bit aligned addresses require address bits 21:3. However, these 64-bit aligned addresses should be programmed to start at a 256-bit DRAM word boundary.

Unlike PEC1, there are no constraints in SoPEC on data organization in DRAM except that all data structures must start on a 256-bit DRAM boundary. If data stored is not a multiple of 256-bits then the last word should be padded.

The CPU subsystem bus supports 32-bit word aligned read and write accesses with variable access timings. The CPU subsystem bus does not currently support byte reads and writes. The PCU only supports 32-bit register reads and writes for the PEP blocks. As the PEP blocks only occupy a subsection of the overall address map and the PCU is explicitly selected by the MMU when a PEP block is being accessed the PCU does not need to perform a decode of the higher-order address bits.

The system wide memory map is shown in FIG. 14.

As outlined above, SoPEC has a requirement to print 1 side every 2 seconds i.e. 30 sides per minute.

Approximately 2 Mbytes of DRAM are reserved for compressed page buffering in SoPEC. If a page is compressed to fit within 2 Mbyte then a complete page can be transferred to DRAM before printing. USB2.0 in high speed mode allows the transfer of 2 Mbyte in less than 40 ms, so data transfer from the host is not a significant factor in print time in this case. For a host PC running in USB 1.1 compatible full speed mode, the transfer time for 2 Mbyte approaches 2 seconds, so the cycle time for full page buffering approaches 4 seconds.

The SoPEC page-expansion blocks support the notion of page banding. The page can be divided into bands and another band can be sent down to SoPEC while the current band is being printed.

Therefore printing can start once at least one band has been downloaded.

The band size granularity should be carefully chosen to allow efficient use of the USB bandwidth and DRAM buffer space. It should be small enough to allow seamless 30 sides per minute printing but not so small as to introduce excessive CPU overhead in orchestrating the data transfer and parsing the band headers. Band-finish interrupts have been provided to notify the CPU of free buffer space. It is likely that the host PC will supervise the band transfer and buffer management instead of the SoPEC CPU.

If SoPEC starts printing before the complete page has been transferred to memory there is a risk of a buffer underrun occurring if subsequent bands are not transferred to SoPEC in time e.g. due to insufficient USB bandwidth caused by another USB peripheral consuming USB bandwidth. A buffer underrun occurs if a line synchronisation pulse is received before a line of data has been transferred to the printhead and causes the print job to fail at that line. If there is no risk of buffer underrun then printing can safely start once at least one band has been downloaded.

If there is a risk of a buffer underrun occurring due to an interruption of compressed page data transfer, then the safest approach is to only start printing once all of the bands have been loaded for a complete page. This means that some latency (dependent on USB speed) will be incurred before printing the first page. Bands for subsequent pages can be downloaded during the printing of the first page as band memory is freed up, so the transfer latency is not incurred for these pages.

A Storage SoPEC, or other memory local to the printer but external to SoPEC, could be added to the system, to provide guaranteed bandwidth data delivery.

The most efficient page banding strategy is likely to be determined on a per page/print job basis and so SoPEC will support the use of bands of any size.

In a system containing more than one SoPECs, the high bandwidth communication path between SoPECs is via USB. Typically, one SoPEC, the ISCMaster, has a USB connection to the host PC, and is responsible for receiving and distributing page data for itself and all other SoPECs in the system. The ISCMaster acts as a USB Device on the host PC's USB bus, and as the USB Host on a USB bus local to the printer.

Any local USB bus in the printer is logically separate from the host PC's USB bus; a SoPEC device does not act as a USB Hub. Therefore the host PC sees the entire printer system as a single USB function.

The SoPEC UHU supports three ports on the printer's USB bus, allowing the direct connection of up to three additional SoPEC devices (or other USB devices). If more than three USB devices need to be connected, two options are available:

-   -   Expand the number of ports on the printer USB bus using a USB         Hub chip.     -   Create one or more additional printer USB busses, using the UHU         ports on other SoPEC devices

FIG. 15 shows these options.

Since the UDU and UHU for a single SoPEC are on logically different USB busses, data flow between them is via the on-chip DRAM, under the control of the SoPEC CPU. There is no direct communication, either at control or data level, between the UDU and the UHU. For example, when the host PC sends compressed page data to a multi-SoPEC system, all the data for all SoPECs must pass via the DRAM on the ISCMaster SoPEC. Any control or status messages between the host and any SoPEC will also pass via the ISCMaster's DRAM.

Further, while the UDU on SoPEC supports multiple USB interfaces and endpoints within a single USB device function, it typically does not have a mechanism to identify at the USB level which SoPEC is the ultimate destination of a particular USB data or control transfer. Therefore software on the CPU needs to redirect data on a transfer-by-transfer basis, either by parsing a header embedded in the USB data, or based on previously communicated control information from the host PC. The software overhead involved in this management adds to the overall latency of compressed page download for a multi-SoPEC system.

The UDU and UHU contain highly configurable DMA controllers that allow the CPU to direct USB data to and from DRAM buffers in a flexible way, and to monitor the DMA for a variety of conditions. This means that the CPU can manage the DRAM buffers between the UDU and the UHU without ever needing to physically move or copy packet data in the DRAM.

This chapter is intended to give an overview of a representative set of scenarios or use cases which SoPEC can perform. SoPEC is by no means restricted to the particular use cases described and not every SoPEC system is considered here.

In this chapter, SoPEC use is described under four headings:

-   -   1) Normal operation use cases.     -   2) Security use cases.     -   3) Miscellaneous use cases.     -   4) Failure mode use cases.

Use cases for both single and multi-SoPEC systems are outlined.

Some tasks may be composed of a number of sub-tasks.

SoPEC operation is broken up into a number of sections which are outlined below. Buffer management in a SoPEC system is normally performed by the host.

Powerup describes SoPEC initialisation following an external reset or the watchdog timer system reset.

A typical powerup sequence is:

-   -   1) Execute reset sequence for complete SoPEC.     -   2) CPU boot from ROM.     -   3) Basic configuration of CPU peripherals, UDU and DIU. DRAM         initialisation. USB Wakeup.     -   4) Download and authentication of program.     -   5) Execution of program from DRAM.     -   6) Retrieve operating parameters from PRINTER_QA and         authenticate operating parameters.     -   7) Download and authenticate any further datasets.

The CPU can put different sections of SoPEC into sleep mode by writing to registers in the CPR block. This can include disabling both the DRAM and the CPU itself, and in some circumstances the UDU as well. Some system state is always stored in the power-safe storage (PSS) block.

Wakeup describes SoPEC recovery from sleep mode with the CPU and DRAM disabled. Wakeup can be initiated by a hardware reset, an event on the device or host USB interfaces, or an event on a GPIO pin.

A typical USB wakeup sequence is:

-   -   1) Execute reset sequence for sections of SoPEC in sleep mode.     -   2) CPU boot from ROM, if CPU-subsystem was in sleep mode.     -   3) Basic configuration of CPU peripherals and DIU, and DRAM         initialisation, if required.     -   4) Download and authentication of program using results in         Power-Safe Storage (PSS).     -   5) Execution of program from DRAM.     -   6) Retrieve operating parameters from PRINTER_QA and         authenticate operating parameters.     -   7) Download and authenticate using results in PSS of any further         datasets (programs).

This sequence is typically performed at the start of a print job following powerup or wakeup:

-   -   1) Check amount of ink remaining via QA chips.     -   2) Download static data e.g. dither matrices, dead nozzle tables         from host to DRAM.     -   3) Check printhead temperature, if required, and configure         printhead with firing pulse profile etc. accordingly.     -   4) Initiate printhead pre-heat sequence, if required.

Buffer management in a SoPEC system is normally performed by the host.

First page, first band download and processing:

-   -   1) The host communicates to the SoPEC CPU over the USB to check         that DRAM space remaining is sufficient to download the first         band.     -   2) The host downloads the first band (with the page header) to         DRAM.     -   3) When the complete page header has been downloaded the SoPEC         CPU processes the page header, calculates PEP register commands         and writes directly to PEP registers or to DRAM.     -   4) If PEP register commands have been written to DRAM, execute         PEP commands from DRAM via PCU.

Remaining bands download and processing:

-   -   1) Check DRAM space remaining is sufficient to download the next         band.     -   2) Download the next band with the band header to DRAM.     -   3) When the complete band header has been downloaded, process         the band header according to whichever band-related register         updating mechanism is being used.     -   1) Wait until at least one band of the first page has been         downloaded.     -   2) Start all the PEP Units by writing to their Go registers, via         PCU commands executed from DRAM or direct CPU writes.     -   3) Print ready interrupt occurs (from PHI).     -   4) Start motor control, if first page, otherwise feed the next         page. This step could occur before the print ready interrupt.     -   5) Drive LEDs, monitor paper status.     -   6) Wait for page alignment via page sensor(s) GPIO interrupt.     -   7) CPU instructs PHI to start producing line syncs and hence         commence printing, or wait for an external device to produce         line syncs.     -   8) Continue to download bands and process page and band headers         for next page.

As for first page download, performed during printing of current page.

When the finished band flags are asserted band related registers in the CDU, LBD, TE need to be re-programmed before the subsequent band can be printed. The finished band flag interrupts the CPU to tell the CPU that the area of memory associated with the band is now free. Typically only 3-5 commands per decompression unit need to be executed.

These registers can be either:

-   -   Reprogrammed directly by the CPU after the band has finished     -   Update automatically from shadow registers written by the CPU         while the previous band was being processed

Alternatively, PCU commands can be set up in DRAM to update the registers without direct CPU intervention. The PCU commands can also operate by direct writes between bands, or via the shadow registers.

Typically during page printing ink usage is communicated to the QA chips.

-   -   1) Calculate ink printed (from PHI).     -   2) Decrement ink remaining (via QA chips).     -   3) Check amount of ink remaining (via QA chips). This operation         may be better performed while the page is being printed rather         than at the end of the page.

These operations are typically performed when the page is finished:

-   -   1) Page finished interrupt occurs from PHI.     -   2) Shutdown the PEP blocks by de-asserting their Go registers. A         typical shutdown order is defined in Table 13. This will set the         PEP Unit state-machines to their idle states without resetting         their configuration registers.     -   3) Communicate ink usage to QA chips, if required.

In a multi-SoPEC system the host generally manages program and compressed page download to all the SoPECs. Inter-SoPEC communication is over local USB links, which will add a latency. The SoPEC with the USB connection to the host is the ISCMaster.

In a multi-SoPEC system one of the SoPECs will be the PrintMaster. This SoPEC must manage and control sensors and actuators e.g. motor control. These sensors and actuators could be distributed over all the SoPECs in the system. An ISCMaster SoPEC may also be the PrintMaster SoPEC.

In a multi-SoPEC system each printing SoPEC will generally have its own PRINTER_QA chip (or at least access to a PRINTER_QA chip that contains the SoPEC's SOPEC_id_key) to validate operating parameters and ink usage. The results of these operations may be communicated to the PrintMaster SoPEC.

In general the ISCMaster may need to be able to:

-   -   Send messages to the ISCSlaves which will cause the ISCSlaves to         send their status to the ISCMaster.     -   Instruct the ISCSlaves to perform certain operations.

As the local USB links represent an insecure interface, commands issued by the ISCMaster are regarded as user mode commands. Supervisor mode code running on the SoPEC CPUs will allow or disallow these commands. The software protocol needs to be constructed with this in mind.

The ISCMaster will initiate all communication with the ISCSlaves.

SoPEC operation is broken up into a number of sections which are outlined below.

System errors and security violations are reported to the SoPEC CPU and host. Software running on the SoPEC CPU or host will then decide what actions to take.

Silverbrook code authentication failure.

-   -   1) Notify host PC of authentication failure.     -   2) Abort print run.

OEM code authentication failure.

-   -   1) Notify host PC of authentication failure.     -   2) Abort print run.

Invalid QA chip(s).

-   -   1) Report to host PC.     -   2) Abort print run.

MMU security violation interrupt.

-   -   1) This is handled by exception handler.     -   2) Report to host PC     -   3) Abort print run.

Invalid address interrupt from PCU.

-   -   1) This is handled by exception handler.     -   2) Report to host PC.     -   3) Abort print run.

Watchdog timer interrupt.

-   -   1) This is handled by exception handler.     -   2) Report to host PC.     -   3) Abort print run.

Host PC does not acknowledge message that SoPEC is about to power down.

-   -   1) Power down anyway.

Printing errors are reported to the SoPEC CPU and host. Software running on the host or SoPEC CPU will then decide what actions to take.

Insufficient space available in SoPEC compressed band-store to download a band.

-   -   1) Report to the host PC.

Insufficient ink to print.

-   -   1) Report to host PC.

Page not downloaded in time while printing.

-   -   1) Buffer underrun interrupt will occur.     -   2) Report to host PC and abort print run.

JPEG decoder error interrupt.

-   -   1) Report to host PC.CPU Subsystem

The CPU block consists of the CPU core, caches, MMU, RDU and associated logic. The principal tasks for the program running on the CPU to fulfill in the system are:

Communications:

-   -   Control the flow of data to and from the USB interfaces to and         from the DRAM     -   Communication with the host via USB     -   Communication with other USB devices (which may include other         SoPECs in the system, digital cameras, additional communication         devices such as ethernet-to-USB chips) when SoPEC is functioning         as a USB host     -   Communication with other devices (utilizing the MMI interface         block) via miscellaneous protocols (including but not limited to         Parallel Port, Generic 68K/i960 CPU interfaces, serial         interfaces Intel SBB, Motorola SPI etc.).     -   Running the USB device drivers     -   Running additional protocol stacks (such as ethernet)         PEP Subsystem Control:     -   Page and band header processing (may possibly be performed on         host PC)     -   Configure printing options on a per band, per page, per job or         per power cycle basis     -   Initiate page printing operation in the PEP subsystem     -   Retrieve dead nozzle information from the printhead and forward         to the host PC or process locally     -   Select the appropriate firing pulse profile from a set of         predefined profiles based on the printhead characteristics     -   Retrieve printhead information (from printhead and associated         serial flash)         Security:     -   Authenticate downloaded program code     -   Authenticate printer operating parameters     -   Authenticate consumables via the PRINTER_QA and INK_QA chips     -   Monitor ink usage     -   Isolation of OEM code from direct access to the system resources         Other:     -   Drive the printer motors using the GPIO pins     -   Monitoring the status of the printer (paper jam, tray empty         etc.)     -   Driving front panel LEDs and/or other display devices     -   Perform post-boot initialisation of the SoPEC device     -   Memory management (likely to be in conjunction with the host PC)     -   Handling higher layer protocols for interfaces implemented with         the MMI     -   Image processing functions such as image scaling, cropping,         rotation, white-balance, color space conversion etc. for         printing images directly from digital cameras (e.g. via         PictBridge application software)     -   Miscellaneous housekeeping tasks

To control the Print Engine Pipeline the CPU is required to provide a level of performance at least equivalent to a 16-bit Hitachi H8-3664 microcontroller running at 16 MHz. An as yet undetermined amount of additional CPU performance is needed to perform the other tasks, as well as to provide the potential for such activity as Netpage page assembly and processing, RIPing etc. The extra performance required is dominated by the signature verification task, direct camera printing image processing functions (i.e. color space conversion) and the USB (host and device) management task. A number of CPU cores have been evaluated and the LEON P1754 is considered to be the most appropriate solution. A diagram of the CPU block is shown in FIG. 16.

The Dotline Writer Unit (DWU) receives 1 dot (6 bits) of color information per cycle from the DNC. Dot data received is bundled into 256-bit words and transferred to the DRAM. The DWU (in conjunction with the LLU) implements a dot line FIFO mechanism to compensate for the physical placement of nozzles in a printhead, and provides data rate smoothing to allow for local complexities in the dot data generate pipeline.

The physical placement of nozzles in the printhead means that in one firing sequence of all nozzles, dots will be produced over several print lines. The printhead consists of up to 12 rows of nozzles, one for each color of odd and even dots. Nozzles rows of the same color are separated by D₁ print lines and nozzle rows of different adjacent colors are separated by D₂ print lines. See FIG. 18 for reference. The first color to be printed is the first row of nozzles encountered by the incoming paper. In the example this is color 0 odd, although is dependent on the printhead type. Paper passes under printhead moving upwards.

Due to the construction limitations the printhead can have nozzles mildly sloping over several lines, or a vertical alignment discontinuity at potentially different horizontal positions per row (D₃). The DWU doesn't need any knowledge of the discontinuities only that it stores sufficient lines in the dot store to allow the LLU to compensate.

FIG. 18 shows a possible vertical misalignment of rows within a printhead segment. There will also be possible vertical and horizontal misalignment of rows between adjacent printhead segments.

The DWU compensates for horizontal misalignment of nozzle rows within printhead segments, and writes data out to half line buffers so that the LLU is able to compensate for vertical misalignments between and within printhead segments. The LLU also compensates for the horizontal misalignment between a printhead segment.

For example if the physical separation of each half row is 80 m equating to D₁=D₂=5 print lines at 1600 dpi. This means that in one firing sequence, color 0 odd nozzles 1-17 will fire on dotline L, color 0 even nozzles 0-16 will fire on dotline L-D₁, color 1 odd nozzles 1-17 will fire on dotline L-D₁-D₂ and so on over 6 color planes odd and even nozzles. The total number of physical lines printed onto over a single line time is given as (0+5+5 . . . +5)+1=11×5+1=56. See FIG. 19 for example diagram.

It is expected that the physical spacing of the printhead nozzles will be 80 m (or 5 dot lines), although there is no dependency on nozzle spacing. The DWU is configurable to allow other line nozzle spacings.

The DWU block is required to compensate for the physical spacing between lines of nozzles. It does this by storing dot lines in a FIFO (in DRAM) until such time as they are required by the LLU for dot data transfer to the printhead interface. Colors are stored separately because they are needed at different times by the LLU. The dot line store must store enough lines to compensate for the physical line separation of the printhead but can optionally store more lines to allow system level data rate variation between the read (printhead feed) and write sides (dot data generation pipeline) of the FIFOs.

A logical representation of the FIFOs is shown in FIG. 20, where N is defined as the optional number of extra half lines in the dot line store for data rate de-coupling.

If the printhead contains nozzles sloping over X lines or a vertical misalignment of Y lines then the DWU must store N>X and N>Y lines in the dotstore to allow the LLU to compensate for the nozzle slope and any misalignment. It is also possible that the effects of a slope, and a vertical misalignment are accumulative, in such cases N>(X+Y).

The DNC and the DWU concept of line lengths can be different. The DNC can be programmed to produce less dots than the DWU expects per line, or can be programmed to produce an odd number of dots (the DWU always expect an even number of dots per line). The DWU produces NozzleSkewPadding more dots than it excepts from the DNC per line. If the DNC is required to produce an odd number of dots, the NozzleSkewPadding value can be adjusted to ensure the output from the DWU is still even. The relationship of line lengths between DWU and DNC must always satisfy: (LineSize+1)*2−NozzleSkewPadding==DncLineLength

For an arbitrary page width of d dots (where d is even), the number of dots per half line is d/2.

For interline spacing of D₂ and inter-color spacing of D₁, with C colors of odd and even half lines, the number of half line storage is (C−1) (D₂+D₁)+D₁.

For N extra half line stores for each color odd and even, the storage is given by (N*C*2). The total storage requirement is ((C−1) (D₂+D₁)+D₁+(N*C*2))*d/2 in bits.

Note that when determining the storage requirements for the dot line store, the number of dots per line is the page width and not necessarily the printhead width. The page width is often the dot margin number of dots less than the printhead width. They can be the same size for full bleed printing.

For example in an A4 page a line consists of 13824 dots at 1600 dpi, or 6912 dots per half dot line. To store just enough dot lines to account for an inter-line nozzle spacing of 5 dot lines it would take 55 half dot lines for color 5 odd, 50 dot lines for color 5 even and so on, giving 55+50+45 . . . 10+5+0=330 half dot lines in total. If it is assumed that N=4 then the storage required to store 4 extra half lines per color is 4×12=48, in total giving 330+48=378 half dot lines. Each half dot line is 6912 dots, at 1 bit per dot give a total storage requirement of 6912 dots×378 half dot lines/8 bits=Approx 319 Kbytes. Similarly for an A3 size page with 19488 dots per line, 9744 dots per half line×378 half dot lines/8=Approx 450 Kbytes.

The potential size of the dot line store makes it unfeasible to be implemented in on-chip SRAM, requiring the dot line store to be implemented in embedded DRAM. This allows a configurable dotline store where unused storage can be redistributed for use by other parts of the system.

Due to construction limitations of the printhead it is possible that nozzle rows within a printhead segment may be misaligned relative to each other by up to 5 dots per half line, which means 56 dot positions over 12 half lines (i.e. 28 dot pairs). Vertical misalignment can also occur but is compensated for in the LLU and not considered here. The DWU is required to compensate for the horizontal misalignment.

Dot data from the HCU (through the DNC) produces a dot of 6 colors all destined for the same physical location on paper. If the nozzle rows in the within a printhead segment are aligned as shown in FIG. 18 then no adjustment of the dot data is needed.

A conceptual misaligned printhead is shown in FIG. 21. The exact shape of the row alignment is arbitrary, although is most likely to be sloping (if sloping, it could be sloping in either direction).

The DWU is required to adjust the shape of the dot streams to take into account the relative horizontal displacement of nozzles rows between 2 adjacent printhead segments. The LLU compensates for the vertical skew between printhead segments, and the vertical and horizontal skew within printhead segments. The nozzle row skew function aligns rows to compensate for the seam between printhead segments (as shown in FIG. 21) and not for the seam within a printhead (as shown in FIG. 18). The DWU nozzle row function results in aligned rows as shown in the example in FIG. 22.

To insert the shape of the skew into the dot stream, for each line we must first insert the dots for non-printable area 1, then the printable area data (from the DNC), and then finally the dots for non-printable area 2. This can also be considered as: first produce the dots for non-printable area 1 for line n, and then a repetition of:

-   -   produce the dots for the printable area for line n (from the         DNC)     -   produce the dots for the non-printable area 2 (for line n)         followed by the dots of non-printable area 1 (for line n+1)

The reason for considering the problem this way is that regardless of the shape of the skew, the shape of non-printable area 2 merged with the shape of non-printable area 1 will always be a rectangle since the widths of non-printable areas 1 and 2 are identical and the lengths of each row are identical. Hence step 2 can be accomplished by simply inserting a constant number (NozzleSkewPadding) of 0 dots into the stream.

For example, if the color n even row non-printable area 1 is of length X, then the length of color n even row non-printable area 2 will be of length NozzleSkewPadding−X. The split between non-printable areas 1 and 2 is defined by the NozzleSkew registers.

Data from the DNC is destined for the printable area only, the DWU must generate the data destined for the non-printable areas, and insert DNC dot data correctly into the dot data stream before writing dot data to the fifos. The DWU inserts the shape of the misalignment into the dot stream by delaying dot data destined to different nozzle rows by the relative misalignment skew amount.

The Line Loader Unit (LLU) reads dot data from the line buffers in DRAM and structures the data into even and odd dot channels destined for the same print time. The blocks of dot data are transferred to the PHI and then to the printhead. FIG. 23 shows a high level data flow diagram of the LLU in context.

The DWU re-orders dot data into 12 separate dot data line FIFOs in the DRAM. Each FIFO corresponds to 6 colors of odd and even data. The LLU reads the dot data line FIFOs and sends the data to the printhead interface. The LLU decides when data should be read from the dot data line FIFOs to correspond with the time that the particular nozzle on the printhead is passing the current line. The interaction of the DWU and LLU with the dot line FIFOs compensates for the physical spread of nozzles firing over several lines at once. For further explanation see Section 32 Dotline Writer Unit (DWU) and Section 34 PrintHead Interface (PHI). FIG. 24 shows the physical relationship between nozzle rows and the line time the LLU starts reading from the dot line store.

A printhead is constructed from printhead segments. One A4 printhead can be constructed from up to 11 printhead segments. A single LLU needs to be capable of driving up to 11 printhead segments, although it may be required to drive less. The LLU will read this data out of FIFOs written by the DWU, one FIFO per half-color.

The PHI needs to send data out over 6 data lines, each data line may be connected to up to two segments. When printing A4 portrait, there will be 11 segments. This means five of the data lines will have two segments connected and one will have a single segment connected (any printhead channel could have a single segment connected). In a dual SoPEC system, one of the SoPECs will be connected to 5 segments, while the other is connected to 6 segments.

Focusing for a moment on the single SoPEC case, SoPEC maintains a data generation rate of 6 bits per cycle throughout the data calculation path. If all 6 data lines broadcast for the entire duration of a line, then each would need to sustain 1 bit per cycle to match SoPECs internal processing rate. However, since there are 11 segments and 6 data lines, one of the lines has only a single segment attached. This data line receives only half as much data during each print line as the other data lines. So if the broadcast rate on a line is 1 bit per cycle, then we can only output at a sustained rate of 5.5 bits per cycle, thus not matching the internal generation rate. These lines therefore need an output rate of at least 6/5.5 bits per cycle.

Due to clock generation limitations in SoPEC the PHI datalines can transport data at 6/5 bits per cycle, slightly faster than required.

While the data line bandwidth is slightly more than is needed, the bandwidth needed is still slightly over 1 bit per cycle, and the LLU data generators that prepare data for them must produce data at over 1 bit per cycle. To this end the LLU will target generating data at 2 bits per cycle for each data line.

The LLU will have 6 data generators. Each data generator will produce the data for either a single segment, or for 2 segments. In cases where a generator is servicing multiple segments the data for one entire segment is generated first before the next segments data is generated. Each data generator will have a basic data production rate of 2 bits per cycle, as discussed above. The data generators need to cater to variable segment width. The data generators will also need to cater for the full range of printhead designs currently considered plausible. Dot data is generated and sent in increasing order.

What has to be dealt with in the LLU is summarized here.

The generators need to be able to cope with segments being vertically offset. This could be due to poor placement and assembly techniques, or due to each printhead segment being placed slightly above or below the previous printhead segment.

They need to be able to cope with the segments being placed at mild slopes. The slopes being discussed and planned for are of the order of 5-10 lines across the width of the printhead (termed Sloped Step).

It is necessary to cope with printhead segments that have a single internal step of 3-10 lines thus avoiding the need for continuous slope. Note the term step is used to denote when the LLU changes the dot line it is reading from in the dot line store. To solve this we will reuse the mild sloping facility, but allow the distance stepped back to be arbitrary, thus it would be several steps of one line in most mild sloping arrangements and one step of several lines in a single step printhead. SoPEC should cope with a broad range of printhead sizes. It is likely that the printheads used will be 1280 dots across. Note this is 640 dots/nozzles per half color.

It is also necessary that the LLU be able to cope with a single internal step, where the step position varies per nozzle row within a segment rather than per segment (termed Single Step).

The LLU can compensate for either a Sloped Step or Single Step, and must compensate all segments in the printhead with the same manner.

Due to construction limitations of the linking printhead it is possible that nozzle rows may be misaligned relative to each other. Odd and even rows, and adjacent color rows may be horizontally misaligned by up to 5 dot positions relative to each other. Vertical misalignment can also occur between printhead segments used to construct the printhead. The DWU compensates for some horizontal misalignment issues (see Section 32.5), and the LLU compensates for the vertical misalignments and some horizontal misalignment.

The vertical skew between printhead segments can be different between any 2 segments. For example the vertical difference between segment A and segment B (Vertical skew AB) and between segment B and segment C (Vertical skew BC) can be different.

The LLU compensates for this by maintaining a different set of address pointers for each segment. The segment offset register (SegDRAMOffset) specifies the number of DRAM words offset from the base address for a segment. It specifies the number of DRAM words to be added to the color base address for each segment, and is the same for all odd colors and even colors within that segment. The SegDotOffset specifies the bit position within that DRAM word to start processing dots, there is one register for all even colors and one for all odd colors within that segment. The segment offset is programmed to account for a number of dot lines, and compensates for the printhead segment mis-alignment. For example in the diagram above the segment offset for printhead segment B is SegWidth+(LineLength*3) in DRAM words.

Vertical skew within a segment can take the form of either a single step of 3-10 lines, or a mild slope of 5-10 lines across the length of the printhead segment. Both types of vertical skew are compensated for by the LLU using the same mechanism, but with different programming.

Within a segment there may be a mild slope that the LLU must compensate for by reading dot data from different parts of the dot store as it produces data for a segment. Every SegSpan number of dot pairs the LLU dot generator must adjust the address pointer by StepOffset. The StepOffset is added to the address pointer but a negative offset can be achieved by setting StepOffset sufficiently large enough to wrap around the dot line store. When a dot generator reaches the end of a segment span and jumps to the new DRAM word specified by the offset, the dot pointer (pointing to the dot within a DRAM word) continues on from the same position it finished. It is possible (and likely) that the span step will not align with a segment edge. The span counter must start at a configured value (ColorSpanStart) to compensate for the mis-alignment of the span step and the segment edge.

The programming of the ColorSpanStart, StepOffset and SegSpan can be easily reprogrammed to account for the single step case.

All segments in a printhead are compensated using the same ColorSpanStart, StepOffset and SegSpan settings, no parameter can be adjusted on a per segment basis.

With each step jump not aligned to a 256-bit word boundary, data within a DRAM word will be discarded. This means that the LLU must have increased DRAM bandwidth to compensate for the bandwidth lost due to data getting discarded.

The LLU is also required to compensate for color row dependant vertical step offset. The position of the step offset is different for each color row and but the amount of the offset is the same per color row. Color dependent vertical skew will be the same for all segments in the printhead.

The color dependant step compensation mechanism is a variation of the sloped and single step mechanisms described earlier. The step offset position within a printhead segment varies per color row. The step offset position is adjusted by setting the span counter to different start values depending on the color row being processed. The step offset is defined as SegSpan−ColorSpanStart[N] where N specifies the color row to process.

In the skewed edge sloped step case it is likely the mechanism will be used to compensate for effects of the shape of the edge of the printhead segment. In the skewed edge single step case it is likely the mechanism will be used to compensate for the shape of the edge of the printhead segment and to account for the shape of the internal edge within a segment.

The LLU is required to compensate for horizontal misalignments between printhead segments. FIG. 28 shows possible misalignment cases.

In order for the LLU to compensate for horizontal misalignment it must deal with 3 main issues

-   -   Swap odd/even dots to even/odd nozzle rows (case 2 and 4)     -   Remove duplicated dots (case 2 and 4)     -   Read dots on a dot boundary rather than a dot pair

In case 2 the second printhead segment is misaligned by one dot. To compensate for the misalignment the LLU must send odd nozzle data to the even nozzle row, and even nozzle data to the odd nozzle row in printhead segment 2. The OddAligned register configures if a printhead segment should have odd/even data swapped, when set the LLU reads even dot data and transmits it to the odd nozzle row (and visa versa).

When data is swapped, nozzles in segment 2 will overlap with nozzles in segment 1 (indicated in FIG. 28), potentially causing the same dot data to be fired twice to the same position on the paper. To prevent this the LLU provides a mechanism whereby the first dots in a nozzle row in a segment are zeroed or prevented from firing. The SegStartDotRemove register configures the number of starting dots (up to a maximum of 3 dots) in a row that should be removed or zeroed out on a per segment basis. For each segment there are 2 registers one for even nozzle rows and one for odd nozzle rows.

Another consequence of nozzle row swapping, is that nozzle row data destined for printhead segment 2 is no longer aligned. Recall that the DWU compensates for a fixed horizontal skew that has no knowledge of odd/even nozzle data swapping. Notice that in Case 2 b in FIG. 28 that odd dot data destined for the even nozzle row of printhead segment 2 must account for the 3 missing dots between the printhead segments, whereas even dot data destined for the odd nozzle row of printhead segment 2 must account for the 2 duplicate dots at the start of the nozzle row. The LLU allows for this by providing different starting offsets for odd and even nozzles rows and a per segment basis. The SegDRAMOffset and SegDotOffset registers have 12 sets of 2 registers, one set per segment, and within a set one register per odd/even nozzle row. The SegDotOffset register allows specification of dot offsets on a dot boundary.

The LLU (in conjunction with sub-line compensation in printhead segments) is required to compensate for sub-line vertical skew between printhead segments.

FIG. 29 shows conceptual example cases to illustrate the sub-line compensation problem. Consider a printhead segment with 10 rows each spaced exactly 5 lines apart. The printhead segment takes 100 us to fire a complete line, 10 us per row. The paper is moving continuously while the segment is firing, so row 0 will fire on line A, row 1 will 10 us later on Line A+0.1 of a line, and so on until to row 9 which is fire 90 us later on line A+0.9 of a line (note this assumes the 5 line row spacing is already compensated for). The resultant dot spacing is shown in case 1A in FIG. 29.

If the printhead segment is constructed with a row spacing of 4.9 lines and the LLU compensates for a row spacing of 5 lines, case 1B will result with all nozzle rows firing exactly on top of each other. Row 0 will fire on line A, row 1 will fire 10 us later and the paper will have moved 0.1 line, but the row separation is 4.9 lines resulting in row 1 firing on line A exactly, (line A+4.9 lines physical row spacing−5 lines due to LLU row spacing compensation+0.1 lines due to 10 us firing delay=line A).

Consider segment 2 that is skewed relative to segment 1 by 0.3 of a line. A normal printhead segment without sub-line adjustment would print similar to case 2A. A printhead segment with sub-line compensation would print similar to case 2B, with dots from all nozzle rows landing on Line A+segment skew (in this case 0.3 of a line).

If the firing order of rows is adjusted, so instead of firing rows 0, 1, 2 . . . 9, the order is 3, 4, 5 . . . 8, 9, 0, 1, 2, and a printhead with no sub-line compensation is used a pattern similar to case 2C will result. A dot from nozzle row 3 will fire at line A+segment skew, row 4 at line A+segment skew+0.1 of a line etc. (note that the dots are now almost aligned with segment 1). If a printhead with sub-line compensation is used, a dot from nozzle row 3 will fire on line A, row 4 will fire on line A and so on to row 9, but rows 0, 1, 2 will fire on line B (as shown in case 2D).

The LLU is required to compensate for normal row spacing (in this case spacing of 5 lines), it needs to also compensate on a per row basis for a further line due to sub-line compensation adjustments in the printhead. In case 2D, the firing pattern and resulting dot locations for rows 0, 1, 2 means that these rows would need to be loaded with data from the following line of a page in order to be printing the correct dot data to the correct position. When the LLU adjustments are applied and a sub-line compensating printhead segment is used a dot pattern as shown in case 2E will result, compensating for the sub-line skew between segment 1 and 2.

The LLU is configured to adjust the line spacing on a per row per segment basis by programming the SegColorRowInc registers, one register per segment, and one bit per row. The specific sub-line placement of each row, and subsequent standard firing order is dependant on the design of the printhead in question. However, for any such firing order, a different ordering can be constructed, like in the above sample, that results in sub-line correction. And while in the example above it is the first three rows which required adjustment it might equally be the last three or even three non-contiguous rows that require different data than normal when this facility is engaged. To support this flexibly the LLU needs to be able to specify for each segment a set of rows for which the data is loaded from one line further into the page than the default programming for that half-color.

The Printhead interface (PHI) accepts dot data from the LLU and transmits the dot data to the printhead, using the printhead interface mechanism. The PHI generates the control and timing signals necessary to load and drive the printhead. A printhead is constructed from a number of printhead segments. The PHI has 6 transmission lines (printhead channel), each line is capable of driving up to 2 printhead segments, allowing a single PHI to drive up to 12 printhead segments. The PHI is capable of driving any combination of 0, 1 or 2 segments on any printhead channel.

The PHI generates control information for transmission to each printhead segment. The control information can be generated automatically by the PHI based on configured values, or can be constructed by the CPU for the PHI to insert into the data stream.

The basic idea of the linking printhead is that we create a printhead from tiles each of which can be fully formed within the reticle. The printheads are linked together as shown in FIG. 58 to form the page-width printhead. For example, an A4/Letter page is assembled from 11 tiles.

The printhead is assembled by linking or butting up tiles next to each other. The physical process used for linking means that wide-format printheads are not readily fabricated (unlike the 21 mm tile). However printers up to around A3 portrait width (12 inches) are expected to be possible.

The nozzles within a single segment are grouped physically to reduce ink supply complexity and wiring complexity. They are also grouped logically to minimize power consumption and to enable a variety of printing speeds, thereby allowing speed/power consumption trade-offs to be made in different product configurations.

Each printhead segment contains a constant number of nozzles per color (currently 1280), divided into half (640) even dots and half (640) odd dots. If all of the nozzles for a single color were fired at simultaneously, the even and odd dots would be printed on different dot-rows of the page such that the spatial difference between any even/odd dot-pair is an exact number of dot lines. In addition, the distance between a dot from one color and the corresponding dot from the next color is also an exact number of dot lines.

The exact distance between even and odd nozzle rows, and between colors will vary between embodiments, so it is preferred that these relationships be programmable with respect to SoPEC.

When 11 segments are joined together to create a 30 ppm printhead, a single SoPEC will connect to them as shown in FIG. 59.

Notice that each phDataOutn lvds pair goes to two adjacent printhead segments, and that each phClkn signal goes to 5 or 6 printhead segments. Each phRstn signal goes to alternate printhead segments.

SoPEC drives phRst0 and phRst1 to put all the segments into reset.

SoPEC then lets phRst1 come out of reset, which means that all the segment 1, 3, 5, 7, and 9 are now alive and are capable of receiving commands.

SoPEC can then communicate with segment 1 by sending commands down phDataOut0, and program the segment 1 to be id 1. It can communicate with segment 3 by sending commands down phDataOut1, and program segment 3 to be id 1. This process is repeated until all segments 1, 3, 5, 7, and 9 are assigned ids of 1. The id only needs to be unique per segment addressed by a given phDataOutn line.

SoPEC can then let phRst0 come out of reset, which means that segments 0, 2, 4, 6, 8, and 10 are all alive and are capable of receiving commands. The default id after reset is 0, so now each of the segments is capable of receiving commands along the same pDataOutn line.

SoPEC needs to be able to send commands to individual printheads, and it does so by writing to particular registers at particular addresses.

The exact relationship between id and register address etc. is yet to be determined, but at the very least it will involve the CPU being capable of telling the PHI to send a command byte sequence down a particular phDataOutn line.

One possibility is that one register contains the id (possibly 2 bits of id). Further, a command may consist of:

-   -   register write     -   register address     -   data

A 10-bit wide fifo can be used for commands in the PHI.

When 11 segments are joined together to create a 60 ppm printhead, the 2 SoPECs will connect to them as shown in FIG. 60 below.

In the 60 ppm case only phClk0 and phRst0 are used (phClk1 and phRst1 are not required). However note that lineSync is required instead. It is possible therefore to reuse phRst1 as a lineSync signal for multi-SoPEC synchronisation. It is not possible to reuse the pins from phClk1 as they are lvds. It should be possible to disable the lvds pads of phClk1 on both SoPECs and phDataOut5 on SoPEC B and therefore save a small amount of power.

This section details various classes of printhead that can be used. With the exception of the PEC1 style slope printhead, SoPEC is designed to be capable of working with each of these printhead types at full 60 ppm printing speed.

The A-chip/A-chip is printhead style consists of identical printhead tiles (type A) assembled in such a way that rows of nozzles between 2 adjacent chips have no vertical misalignment.

The most ideal format for this kind of printhead from a data delivery point of view is a rectangular join between two adjacent printheads, as shown in FIG. 61. However due to the requirement for dots to be overlapping, a rectangular join results in a it results in a vertical stripe of white down the join section since no nozzle can be in this join region. A white stripe is not acceptable, and therefore this join type is not acceptable.

FIG. 62 shows a sloping join, and FIG. 63 is a zoom in of a single color component, illustrating the way in which there is no visible join from a printing point of view (i.e. the problem seen in FIG. 61 has been solved).

Due to a variety of factors (including ink sealing) it may not be possible to have perfect vertical alignment. To create more space between the nozzles, A-chips can be joined with a growing vertical offset, as shown in FIG. 64.

The growing offset comes from the vertical offset between two adjacent tiles. This offset increases with each join. For example, if the offset were 7 lines per join, then an 11 segment printhead would have a total of 10 joins, and 70 lines.

To supply print data to the printhead for a growing offset arrangement, the print data for the relevant lines must be present. A simplistic solution of simply holding the entire line of data for each additional line required leads to increased line store requirements. For example, an 11 segment×1280-dot printhead requires an additional 11×1280-dots×6-colors per line i.e. 10.3125 Kbytes per line. 70 lines requires 722 Kbytes of additional storage. Considering SoPEC contains only 2.5 MB total storage, an additional 722 Kbytes just for the offset component is not desirable. Smarter solutions require storage of smaller parts of the line, but the net effect is the same: increased storage requirements to cope with the growing vertical offset.

The problem of a growing offset is that a number of additional lines of storage need to be kept, and this number increases proportional to the number of joins i.e. the longer the printhead the more lines of storage are required.

However, we can place each chip on a mild slope to achieve a constant number of printlines regardless of the number of joins. The arrangement is similar to that used in PEC1, where the printheads are sloping. The difference here is that each printhead is only mildly sloping, for example so that the total number of lines gained over the length of the printhead is 7. The next printhead can then be placed offset from the first, but this offset would be from the same base. i.e. a printhead line of nozzles starts addressing line n, but moves to different lines such that by the end of the line of nozzles, the dots are 7 dotlines distant from the startline. This means that the 7-line offset required by a growing-offset printhead can be accommodated.

The arrangement is shown in FIG. 65.

Note also, that in this example, the printhead segments are vertically aligned (as in PEC1). It may be that the slope can only be a particular amount, and that growing offset compensates for additional differences—i.e. the segments could in theory be misaligned vertically.

The question then arises as to how much slope must be compensated for at 60 ppm speed. Basically—as much as can comfortably handled without too much logic. However, amounts like 1 in 256 (i.e. 1 in 128 with respect to a half color), or 1 in 128 (i.e. 1 in 64 with respect to a half color) must be possible. Greater slopes and weirder slopes (e.g. 1 in 129 with respect to a half color) must be possible, but with a sacrifice of speed i.e. SoPEC must be capable even if it is a slower print.

Note also that the nozzles are aligned, but the chip is placed sloped. This means that when horizontal lines are attempted to be printed and if all nozzles were fired at once, the effect would be lots of sloped lines. However, if the nozzles are fired in the correct order relative to the paper movement, the result is a straight line for n dots, then another straight line for n dots 1 line up.

The PEC1 style slope is the physical arrangement used by printhead segments addressed by PEC1. Note that SoPEC is not expected to work at 60 ppm speed with printheads connected in this way. However it is expected to work and is shown here for completeness, and if tests should prove that there is no working alternative to the 21 mm tile, then SoPEC will require significant reworking to accommodate this arrangement at 60 ppm.

In this scheme, the segments are joined together by being placed on an angle such that the segments fit under each other, as shown in FIG. 66. The exact angle will depend on the width of the Memjet segment and the amount of overlap desired, but the vertical height is expected to be in the order of 1 mm, which equates to 64 dot lines at 1600 dpi.

FIG. 67 shows more detail of a single segment in a multi-segment configuration, considering only a single row of nozzles for a single color plane. Each of the segments can be considered to produce dots for multiple sets of lines. The leftmost d nozzles (d depends on the angle that the segment is placed at) produce dots for line n, the next d nozzles produce dots for line n−1, and so on.

The A-chip/A-chip with inter-line slope compensation is effectively the same as described above except that the nozzles are physically arranged inside the printhead to compensate for the nozzle firing order given the desire to spread the power across the printhead. This means that one nozzle and its neighbor can be vertically separated on the printhead by 1 printline. i.e. the nozzles don't line up across the printhead. This means a jagged effect on printed “horizontal lines” is avoided, while achieving the goal of averaging the power.

The arrangement of printheads is the same as that shown in FIG. 65. However the actual nozzles are slightly differently arranged, as illustrated via magnification in FIG. 68.

Another possibility is to have two kinds of printing chips: an A-type and a B-type. The two types of chips have different shapes, but can be joined together to form long printheads. A parallelogram is formed when the A-type and B-type are joined.

The two types are joined together as shown in FIG. 69.

Note that this is not a growing offset. The segments of a multiple-segment printhead have alternating fixed vertical offset from a common point, as shown in FIG. 70.

If the vertical offset from a type-A to a type-B printhead were n lines, the entire printhead regardless of length would have a total of n lines additionally required in the line store. This is certainly a better proposition than a growing offset).

However there are many issues associated with an A-chip/B-chip printhead. Firstly, there are two different chips i.e. an A-chip, and a B-chip. This means 2 masks, 2 developments, verification, and different handling, sources etc. It also means that the shape of the joins are different for each printhead segment, and this can also imply different numbers of nozzles in each printhead. Generally this is not a good option.

The general linking concept illustrated in the A-chip/B-chip above can be incorporated into a single printhead chip that contains the A-B join within the single chip type.

This kind of joining mechanism is referred to as the A-B chip since it is a single chip with A and B characteristics. The two types are joined together as shown in FIG. 71.

This has the advantage of the single chip for manipulation purposes.

The A-B chip with printhead compensation kind of printhead is where we push the A-B chip discontinuity as far along the printhead segment as possible—right to the edge. This maximises the A part of the chip, and minimizes the B part of the chip. If the B part is small enough, then the compensation for vertical misalignment can be incorporated on the printhead, and therefore the printhead appears to SoPEC as if it was a single typeA chip. This only makes sense if the B part is minimized since printhead real-estate is more expensive at 0.35 microns rather than on SoPEC at 0.18 microns.

The arrangement is shown in FIG. 72.

Note that since the compensation is accomplished on the printhead, the direction of paper movement is fixed with respect to the printhead. This is because the printhead is keeping a history of the data to apply at a later time and is only required to keep the small amount of data from the B part of the printhead rather than the A part.

Within reason, some of the various linking methods can be combined. For example, we may have a mild slope of 5 over the printhead, plus an on-chip compensation for a further 2 lines for a total of 7 lines between type A chips. The mild slope of 5 allows for a 1 in 128 per half color (a reasonable bandwidth increase), and the remaining 2 lines are compensated for in the printheads so do not impact bandwidth at all.

However we can assume that some combinations make less sense. For example, we do not expect to see an A-B chip with a mild slope.

SoPEC also caters for printheads and printhead modules that have redundant nozzle rows. The idea is that for one print line, we fire from nozzles in row x, in the next print line we fire from the nozzles in row y, and the next print line we fire from row x again etc. Thus, if there are any defective nozzles in a given row, the visual effect is halved since we only print every second line from that row of nozzles. This kind of redundancy requires SoPEC to generate data for different physical lines instead of consecutive lines, and also requires additional dot line storage to cater for the redundant rows of nozzles.

Redundancy can be present on a per-color basis. For example, K may have redundant nozzles, but C, M, and Y have no redundancy.

In the preferred form, we are concerned with redundant row pairs, i.e. rows 0+1 always print odd and even dots of the same colour, so redundancy would require say rows 0+1 to alternate with rows 2+3.

In other embodiments, one or more redundant rows can also be used to implement per-nozzle replacement in the case of one or more dead nozzles. In this case, the nozzles in the redundant row only print dots for positions where a nozzle in the main row is defective. This may mean that only a relatively small numbers of nozzles in the redundant row ever print, but this setup has the advantage that two failed printhead modules (ie, printhead modules with one or more defective nozzles) can be used, perhaps mounted alongside each other on the one printhead, to provide gap-free printing. Of course, if this is to work correctly, it is important to select printhead modules that have different defective nozzles, so that the operative nozzles in each printhead module can compensate for the dead nozzle or nozzles in the other.

Whilst probably of questionable commercial usefulness, it is also possible to have more than one additional row for redundancy per color. It is also possible that only some rows have redundant equivalents. For example, black might have a redundant row due to its high visibility on white paper, whereas yellow might be a less likely candidate since a defective yellow nozzle is much less likely to produce a visually objectionable result.

To accomplish the various printhead requirements, the DWU specification must be updated. The changes to the DWU are minor and basically result in a simplification of the unit.

The preferred data skew block copes with a maximum skew of 24 dots by the use of 12 12-bit shift registers (one shift register per half-color). This can be improved where desired; to cope with a 64 dot skew (i.e. 12 32-bit shift registers), for example.

The DWU currently has an ability to write data in an increasing sense (ascending addresses) or in a decreasing sense (descending addresses). So for example, registers such as ColorLineSense specify direction for a particular half-color.

The DWU now only needs to deal with increasing sense only.

To accomplish the various printhead requirements, the LLU specification must be updated. The LLU needs to provide data for up to eleven printhead segments. It will read this data out of fifos written by the DWU, one fifo per half-color.

The PHI needs to send data out over 6 data lines, where each data line may be connected to up to two segments. When printing A4 portrait, there will be 11 segments. This means five of the datalines will have two segments connected and one will have a single segment connected. (I say ‘one’ and not ‘the last’, since the singly used line may go to either end, or indeed into the middle of the page.) In a dual SoPEC system, one of the SoPECs will be connected to 5 segments, while the other is connected to 6 segments.

Focusing for a moment on the single SoPEC case. Sopec maintains a data generation rate of 6 bpc throughout the data calculation path. If all six data lines broadcast for the entire duration of a line, then each would need to sustain 1 bpc to match SoPEC's internal processing rate. However, since there are eleven segments and six data lines, one of the lines has only a single segment attached. This dataline receives only half as much data during each print line as the other datalines. So if the broadcast rate on a line is 1 bpc, then we can only output at a sustained rate of 5.5 bpc, thus not matching the internal generation rate. These lines therefore need an output rate of at least 6/5.5 bpc. However, from an earlier version of the plan for the PHI and printheads the dataline is set to transport data at 6/5 bpc, which is also a convenient clock to generate and thus has been retained.

So, the datalines carry over one bit per cycle each. While their bandwidth is slightly more than is needed, the bandwidth needed is still slightly over 1 bpc, and whatever prepares the data for them must produce the data at over 1 bpc. To this end the LLU will target generating data at 2 bpc for each data line.

The LLU will have six data generators. Each data generator will produce the data from either a single segment, or two segments. In those cases where a generator is servicing multiple segments the data for one entire segment is generated before the next segment is generated. Each data generator will have a basic data production rate of 2 bpc, as discussed above. The data generators need to cater to variable segment width. The data generators will also need to cater for the full range of printhead designs currently considered plausible. Dot data is generated and sent in increasing order.

The generators need to be able to cope with segments being vertically offset relative to each other. This could be due to poor placement and assembly techniques, or due to each printhead being placed slightly above or below the previous printhead.

They need to be able to cope with the segments being placed at mild slopes. The slopes being discussed and thus planned for are on the order of 5-10 lines across the width of the printhead.

It is necessary to cope with printhead that have a single internal step of 3-10 lines thus avoiding the need for continuous slope. To solve this we will reuse the mild sloping facility, but allow the distance stepped back to be arbitrary, thus it would be several steps of one line in most mild sloping arrangements and one step of several lines in a single step printhead.

SoPEC should cope with a broad range of printhead sizes. It is likely that the printheads used will be 1280 dots across. Note this is 640 dots/nozzles per half color.

A dot generator will process zero or one or two segments, based on a two bit configuration. When processing a segment it will process the twelve half colors in order, color zero even first, then color zero odd, then color 1 even, etc. The LLU will know how long a segments is, and we will assume all segments are the same length.

To process a color of a segment the generator will need to load the correct word from dram. Each color will have a current base address, which is a pointer into the dot fifo for that color. Each segment has an address offset, which is added to the base address for the current color to find the first word of that colour. For each generator we maintain a current address value, which is operated on to determine the location future reads occur from for that segment. Each segment also has a start bit index associated with it that tells it where in the first word it should start reading data from.

A dot generator will hold a current 256 bit word it is operating on. It maintains a current index into that word. This bit index is maintained for the duration of one color (for one segment), it is incremented whenever data is produced and reset to the segment specified value when a new color is started. 2 bits of data are produced for the PHI each cycle (subject to being ready and handshaking with the PHI).

From the start of the segment each generator maintains a count, which counts the number of bits produced from the current line. The counter is loaded from a start-count value (from a table indexed by the half-color being processed) that is usually set to 0, but in the case of the A-B printhead, may be set to some other non-zero value. The LLU has a slope span value, which indicates how many dots may be produced before a change of line needs to occur. When this many dots have been produced by a dot generator, it will load a new data word and load 0 into the slope counter. The new word may be found by adding a dram address offset value held by the LLU. This value indicates the relative location of the new word; the same value serves for all segment and all colours. When the new word is loaded, the process continues from the current bit index, if bits 62 and 63 had just been read from the old word (prior to slope induced change) then bits 64 and 65 would be used from the newly loaded word.

When the current index reaches the end of the 256 bits current data word, a new word also needs to be loaded. The address for this value can be found by adding one to the current address.

It is possible that the slope counter and the bit index counter will force a read at the same time. In this case the address may be found by adding the slope read offset and one to the current address.

Observe that if a single handshaking is use between the dot generators and the PHI then the slope counter as used above is identical between all 6 generators, i.e. it will hold the same counts and indicate loads at the same times. So a single slope counter can be used. However the read index differs for each generator (since there is a segment configured start value. This means that when a generator encounters a 256-bit boundary in the data will also vary from generator to generator.

After all of the generators have calculated data for all of their segments the LLU should advance a line. This involves signalling the consumption to the DWU, and incrementing all the base address pointers for each color. This increment will generally be done by adding an address offset the size of a line of data. However, to support a possible redundancy model for the printheads, we may need to get alternate lines from different offsets in the fifo. That is, we may print alternate lines on the page from different sets of nozzles in the print head. This is presented as only a single line of nozzles to the PHI and LLU, but the offset of that line with respect to the leading edge of the printhead changes for alternating line. To support this incrementing the LLU stores two address offsets. These offsets are applied on alternate lines. In the normal case both these offsets will simply be programmed to the same value, which will equate to the line size.

The fill level remains as currently described in 31.7.5.

The LLU allows the current base addresses for each color to be writeable by the CPU. These registers will then be set to point to appropriate locations with respect to the starting location used by the DWU, and the design of the printhead in question.

The PHI has six input data lines and it needs to have a local buffer for this data. The data arrives at 2 bits per cycle, needs to be stored in multiples of 8 bits for exporting, and will need to buffer at least a few of these bytes to assist the LLU, by making its continuous supply constraints much weaker.

The PHI accepts data from the LLU, and transmits the data to the printheads. Each printhead is constructed from a number of printhead segments. There are six transmission lines, each of which can be connected to two printhead segments, so up to 12 segments may be addressed. However, for A4 printing, only 11 segments are needed, so in a single SOPEC system, 11 segments will be connected. In a dual SOPEC system, each SOPEC will normally be connect to 5 or 6 segments. However, the PHI should cater for any arrangement of segments off its data lines.

Each data line performs 8b10b encoding. When transmitting data, this converts 8 bits of data to a 10 bit symbol for transmission. The encoding also support a number of Control characters, so the symbol to be sent is specified by a control bit and 8 data bits. When processing dot data, the control bit can be inferred to be zero. However, when sending command strings or passing on CPU instructions or writes to the printhead, the PHI will need to be given 9 bit values, allowing it to determine what to do with them.

The PHI accepts six 2-bit data lines from the LLU. These data lines can all run off the same enable and if so the PHI will only need to produce a single ready signal (or which fine grained protocol is selected). The PHI collects the 2-bit values from each line, and compiles them into 8-bit values for each line. These 8 bit values are store in a short fifo, and eventually fed to the encoder for transmission to printheads. There is a fixed mapping between the input lines and the output lines. The line are label 0 to 5 and they address segments 0 to 11. (0->[0,1] and 1->[2,3]).

The connection requirements of the printheads are as follows. Each printhead has 1 LVDS clk input, 1 LVDS data input, 1 RstL input and one Data out line. The data out lines will combined to a single input back into the SOPEC (probably via the GPIO). The RstL needs to be driven by the board, so the printhead reset on power-up, but should also be drivable by SOPEC (thus supporting differentiation for the printheads, this would also be handled by GPIOs, and may require 2 of them.

The data is transmitted to each printhead segment in a specified order. If more than one segment is connected to a given data line, then the entire data for one segment will be transmitted, then the data for the other segment.

For a particular segment, a line consists of a series of nozzle rows. These consist of a control sequence to start each color, followed by the data for that row of nozzles. This will typically be 80 bytes. The PHI is not told by the LLU when a row has ended, or when a line has ended, it maintains a count of the data from the LLU and compares it to a length register. If the LLU does not send used colors, the PHI also needs to know which colors aren't used, so it can respond appropriately. To avoid padding issues the LLU will always be programmed to provide a segment width that is a multiple of 8 bits. After sending all of the lines, the PHI will wait for a line sync pulse (from the GPIO) and, when it arrives, send a line sync to all of the printheads. Line syncs handling has changed from PEC1 and will be described further below. It is possible that in addition to this the PHI may be required to tell the printhead the line sync period, to assist it in firing nozzles at the correct rate.

To write to a particular printhead the PHI needs to write the message over the correct line, and address it to the correct target segment on that line. Each line only supports two segments. They can be addressed separately or a broadcast address can be used to address them both.

The line sync and if needed the period reporting portion of each line can be broadcast to every printhead, so broadcast address on every active line. The nozzle data portion needs to be line specific.

Apart from these line related messages, SOPEC also needs to send other commands to the printheads. These will be register read and write commands. The PHI needs to send these to specific segments or broadcast them, selected on a case by case basis. This is done by providing a data path from the CPU to the printheads via the PHI. The PHI holds a command stream the CPU has written, and sends these out over the data lines. These commands are inserted into the nozzle data streams being produced by the PHI, or into the gap between line syncs and the first nozzle line start. Each command terminates with a resume nozzle data instruction.

CPU instructions are inserted into the dot data stream to the printhead. Sometimes these instructions will be for particular printheads, and thus go out over single data line. If the LLU has a single handshaking line then the benefit of stalling only on will be limited to the depth of the fifo of data coming from the LLU. However there if a number of short commands are sent to different printheads they could effectively mask each other by taking turns to load the fifo corresponding to that segment. In some cases, the benefit in time may not warrant the additional complexity, since with single handshaking and good cross segment synchronisation, all the fifo logic can be simplified and such register writes are unlikely to be numerous. If there is multiple handshaking with the LLU, then stalling a single line while the CPU borrows it is simple and a good idea.

A linking printhead is constructed from linking printhead ICs, placed on a substrate containing ink supply holes. An A4 pagewidth printer used 11 linking printhead ICs. Each printhead is placed on the substrate with reference to positioning fidicuals on the substrate. FIG. 73 shows the arrangement of the printhead ICs (also known as segments) on a printhead. The join between two ICs is shown in detail. The left-most nozzles on each row are dropped by 10 line-pitches, to allow continuous printing across the join. FIG. 73 also introduces some naming and co-ordinate conventions used throughout this document.

FIG. 73 shows the anticipated first generation linking printhead nozzle arrangements, with 10 nozzle rows supporting five colours. The SoPEC compensation mechanisms are general enough to cover other nozzle arrangements.

Printheads ICs may be misplaced relative to their ideal position. This misplacement may include any combination of:

-   -   x offset     -   y offset     -   yaw (rotation around z)     -   pitch (rotation around y)     -   roll (rotation around z)

In some cases, the best visual results are achieved by considering relative misplacement between adjacent ICs, rather than absolute misplacement from the substrate. There are some practical limits to misplacement, in that a gross misplacement will stop the ink from flowing through the substrate to the ink channels on the chip.

Correcting for misplacement obviously requires the misplacement to be measured. In general this may be achieved directly by inspection of the printhead after assembly, or indirectly by scanning or examining a printed test pattern.

SoPEC can compensate for misplacement of linking chips in the X-direction, but only snapped to the nearest dot. That is, a misplacement error of less than 0.5 dot-pitches or 7.9375 microns is not compensated for, a misplacement more that 0.5 dot-pitches but less than 1.5 dot-pitches is treated as a misplacement of 1 dot-pitch, etc.

Uncompensated X misplacement can result in three effects:

-   -   printed dots shifted from their correct position for the entire         misplaced segment     -   missing dots in the overlap region between segments.     -   duplicated dots in the overlap region between segments.

SoPEC can correct for each of these three effects.

In preparing line data to be printed, SoPEC buffers in memory the dot data for a number of lines of the image to be printed. Compensation for misplacement generally involves changing the pattern in which this dot data is passed to the printhead ICs.

SoPEC uses separate buffers for the even and odd dots of each colour on each line, since they are printed by different printhead rows. So SoPEC's view of a line at this stage is as (up to) 12 rows of dots, rather than (up to) 6 colours. Nominally, the even dots for a line are printed by the lower of the two rows for that colour on the printhead, and the odd dots are printed by the upper row (see FIG. 73). For the current linking printhead IC, there are 640 nozzles in row. Each row buffer for the full printhead would contain 640×11 dots per line to be printed, plus some padding if required.

In preparing the image, SoPEC can be programmed in the DWU module to precompensate for the fact that each row on the printhead IC is shifted left with respect to the row above. In this way the leftmost dot printed by each row for a colour is the same offset from the start of a row buffer. In fact the programming can support arbitrary shapes for the printhead IC.

SoPEC has independent registers in the LLU module for each segment that determine which dot of the prepared image is sent to the left-most nozzle of that segment. Up to 12 segments are supported. With no misplacement, SoPEC could be programmed to pass dots 0 to 639 in a row to segment 0, dots 640 to 1279 in a row to segment 1, etc.

If segment 1 was misplaced by 2 dot-pitches to the right, SoPEC could be adjusted to pass to dots 641 to 1280 of each row to segment 1 (remembering that each row of data consists entirely of either odd dots or even dots from a line, and that dot 1 on a row is printed two dot positions away from dot 0). This means the dots are printed in the correct position overall. This adjustment is based on the absolute placement of each printhead IC. Dot 640 is not printed at all, since there is no nozzle in that position on the printhead (see Section 3.1.2 for more detail on compensation for missing dots).

A misplacement of an odd number of dot-pitches is more problematic, because it means that the odd dots from the line now need to be printed by the lower row of a colour pair, and the even dots by the upper row of a colour pair on the printhead segment. Further, swapping the odd and even buffers interferes with the precompensation. This results in the position of the first dot to be sent to a segment being different for odd and even rows of the segment. SoPEC addresses this by having independent registers in the LLU to specify the first dot for the odd and even rows of each segment, i.e. 2×12 registers. A further register bit determines whether dot data for odd and even rows should be swapped on a segment by segment basis.

FIG. 74 shows the detailed alignment of dots at the join between two printhead ICs, for various cases of misplacement, for a single colour.

The effects at the join depend on the relative misplacement of the two segments. In the ideal case with no misplacement, the last 3 nozzles of upper row of the segment N interleave with the first three nozzles of the lower row of segment N+1, giving a single nozzle (and so a single printed dot) at each dot-pitch.

When segment N+1 is misplaced to the right relative to segment N (a positive relative offset in X), there are some dot positions without a nozzle, i.e. missing dots. For positive offsets of an odd number of dot-pitches, there may also be some dot positions with two nozzles, i.e. duplicated dots. Negative relative offsets in X of segment N+1 with respect to segment N are less likely, since they would usually result in a collision of the printhead ICs, however they are possible in combination with an offset in Y. A negative offset will always cause duplicated dots, and will cause missing dots in some cases. Note that the placement and tolerances can be deliberately skewed to the right in the manufacturing step to avoid negative offsets.

Where two nozzles occupy the same dot position, the corrections described in Section 3.1.1 will result in SoPEC reading the same dot data from the row buffer for both nozzles. To avoid printing this data twice SoPEC has two registers per segment in the LLU that specify a number (up to 3) of dots to suppress at the start of each row, one register applying to even dot rows, one to odd dot rows.

SoPEC compensates for missing dots by add the missing nozzle position to its dead nozzle map. This tells the dead nozzle compensation logic in the DNC module to distribute the data from that position into the surrounding nozzles, before preparing the row buffers to be printed.

SoPEC can compensate for misplacement of printhead ICs in the Y-direction, but only snapped to the nearest 0.1 of a line. Assuming a line-pitch of 15.875 microns, if an IC is misplaced in Y by 0 microns, SoPEC can print perfectly in Y. If an IC is misplaced by 1.5875 microns in Y, then we can print perfectly. If an IC is misplaced in Y by 3.175 microns, we can print perfectly. But if an IC is misplaced by 3 microns, this is recorded as a misplacement of 3.175 microns (snapping to the nearest 0.1 of a line), and resulting in a Y error of 0.175 microns (most likely an imperceptible error).

Uncompensated Y misplacement results in all the dots for the misplaced segment being printed in the wrong position on the page.

SoPEC's compensation for Y misplacement uses two mechanism, one to address whole line-pitch misplacement, and another to address fractional line-pitch misplacement. These mechanisms can be applied together, to compensate for arbitrary misplacements to the nearest 0.1 of a line.

The buffers are used to hold dot data to be printed for each row. These buffers contain dot data for multiple lines of the image to be printed. Due to the physical separation of nozzle rows on a printhead IC, at any time different rows are printing data from different lines of the image.

For a printhead on which all ICs are ideally placed, row 0 of each segment is printing data from the line N of the image, row 1 of each segment is printing data from row N-M of the image etc. where N is the separation of rows 0 and 1 on the printhead. Separate SoPEC registers in the LLU for each row specify the designed row separations on the printhead, so that SoPEC keeps track of the “current” image line being printed by each row.

If one segment is misplaced by one whole line-pitch, SoPEC can compensate by adjusting the line of the image being sent to each row of that segment. This is achieved by adding an extra offset on the row buffer address used for that segment, for each row buffer. This offset causes SoPEC to provide the dot data to each row of that segment from one line further ahead in the image than the dot data provided to the same row on the other segments. For example, when the correctly placed segments are printing line N of an image with row 0, line N-M of the image with row 1, etc, then the misplaced segment is printing line N+1 of the image with row 0, line N-M+1 of the image with row 1, etc.

SoPEC has one register per segment to specify this whole line-pitch offset. The offset can be multiple line-pitches, compensating for multiple lines of misplacement. Note that the offset can only be in the forward direction, corresponding to a negative Y offset. This means the initial setup of SoPEC must be based on the highest (most positive) Y-axis segment placement, and the offsets for other segments calculated from this baseline. Compensating for Y displacement requires extra lines of dot data buffering in SoPEC, equal to the maximum relative Y offset (in line-pitches) between any two segments on the printhead. For each misplaced segment, each line of misplacement requires approximately 640×10 or 6400 extra bits of memory.

Compensation for fractional line-pitch displacement of a segment is achieved by a combination of SoPEC and printhead IC fire logic.

The nozzle rows in the printhead are positioned by design with vertical spacings in line-pitches that have a integer and fractional component. The fractional components are expressed relative to row zero, and are always some multiple of 0.1 of a line-pitch. The rows are fired sequentially in a given order, and the fractional component of the row spacing matches the distance the paper will move between one row firing and the next. FIG. 75 shows the row position and firing order on the current implementation of the printhead IC. Looking at the first two rows, the paper moves by 0.5 of a line-pitch between the row 0 (fired first) and row 1 (fired sixth). is supplied with dot data from a line 3 lines before the data supplied to row 0. This data ends up on the paper exactly 3 line-pitches apart, as required.

If one printhead IC is vertically misplaced by a non-integer number of line-pitches, row 0 of that segment no longer aligns to row 0 of other segments. However, to the nearest 0.1 of a line, there is one row on the misplaced segment that is an integer number of line-pitches away from row 0 of the ideally placed segments. If this row is fired at the same time as row 0 of the other segments, and it is supplied with dot data from the correct line, then its dots will line up with the dots from row 0 of the other segments, to within a 0.1 of a line-pitch. Subsequent rows on the misplaced printhead can then be fired in their usual order, wrapping back to row 0 after row 9. This firing order results in each row firing at the same time as the rows on the other printheads closest to an integer number of line-pitches away.

FIG. 76 shows an example, in which the misplaced segment is offset by 0.3 of a line-pitch. In this case, row 5 of the misplaced segment is exactly 24.0 line-pitches from row 0 of the ideal segment. Therefore row 5 is fired first on the misplaced segment, followed by row 7, 9, 0 etc. as shown. Each row is fired at the same time as the a row on the ideal segment that is an integer number of lines away. This selection of the start row of the firing sequence is controlled by a register in each printhead IC.

SoPEC's role in the compensation for fractional line-pitch misplacement is to supply the correct dot data for each row. Looking at FIG. 76, we can see that to print correct, row 5 on the misplaced printhead needs dot data from a line 24 lines earlier in the image than the data supplied to row 0. On the ideal printhead, row 5 needs dot data from a line 23 lines earlier in the image than the data supplied to row 0. In general, when a non-default start row is used for a segment, some rows for that segment need their data to be offset by one line, relative to the data they would receive for a default start row. SoPEC has a register in LLU for each row of each segment, that specifies whether to apply a one line offset when fetching data for that row of that segment.

This kind of erroneous rotational displacement means that all the nozzles will end up pointing further up the page in Y or further down the page in Y. The effect is the same as a Y misplacement, except there is a different Y effect for each media thickness (since the amount of misplacement depends on the distance the ink has to travel).

In some cases, it may be that the media thickness makes no effective visual difference to the outcome, and this form of misplacement can simply be incorporated into the Y misplacement compensation. If the media thickness does make a difference which can be characterised, then the Y misplacement programming can be adjusted for each print, based on the media thickness.

It will be appreciated that correction for roll is particularly of interest where more than one printhead module is used to form a printhead, since it is the discontinuities between strips printed by adjacent modules that are most objectionable in this context.

In this rotation, one end of the IC is further into the substrate than the other end. This means that the printing on the page will be dots further apart at the end that is further away from the media (i.e. less optical density), and dots will be closer together at the end that is closest to the media (more optical density) with a linear fade of the effect from one extreme to the other. Whether this produces any kind of visual artifact is unknown, but it is not compensated for in SoPEC.

This kind of erroneous rotational displacement means that the nozzles at one end of a IC will print further down the page in Y than the other end of the IC. There may also be a slight increase in optical density depending on the rotation amount.

SoPEC can compensate for this by providing first order continuity, although not second order continuity in the preferred embodiment. First order continuity (in which the Y position of adjacent line ends is matched) is achieved using the Y offset compensation mechanism, but considering relative rather than absolute misplacement. Second order continuity (in which the slope of the lines in adjacent print modules is at least partially equalised) can be effected by applying a Y offset compensation on a per pixel basis. Whilst one skilled in the art will have little difficulty deriving the timing difference that enables such compensation, SoPEC does not compensate for it and so it is not described here in detail.

FIG. 77 shows an example where printhead IC number 4 is be placed with yaw, is shown in FIG. 77, while all other ICs on the printhead are perfectly placed. The effect of yaw is that the left end of segment 4 of the printhead has an apparent Y offset of −1 line-pitch relative to segment 3, while the right end of segment 4 has an apparent Y offset of 1 line-pitch relative to segment 5.

To provide first-order continuity in this example, the registers on SoPEC would be programmed such that segments 0 to 3 have a Y offset of 0, segment 4 has a Y offset of −1, and segments 5 and above have Y offset of −2. Note that the Y offsets accumulate in this example—even though segment 5 is perfect aligned to segment 3, they have different Y offsets programmed.

It will be appreciated that some compensation is better than none, and it is not necessary in all cases to perfectly correct for roll and/or yaw. Partial compensation may be adequate depending upon the particular application. As with roll, yaw correction is particularly applicable to multi-module printheads, but can also be applied in single module printheads. The printhead will be designed for 5 colors. At present the intended use is:

-   -   cyan     -   magenta     -   yellow     -   black     -   infra-red

However the design methodology must be capable of targeting a number other than 5 should the actual number of colors change. If it does change, it would be to 6 (with fixative being added) or to 4 (with infra-red being dropped).

The printhead chip does not assume any particular ordering of the 5 colour channels. The printhead will contain 1280 nozzles of each color −640 nozzles on one row firing even dots, and 640 nozzles on another row firing odd dots. This means 11 linking printheads are required to assemble an A4/Letter printhead.

However the design methodology must be capable of targeting a number other than 1280 should the actual number of nozzles per color change. Any different length may need to be a multiple of 32 or 64 to allow for ink channel routing.

The printhead will target true 1600 dpi printing. This means ink drops must land on the page separated by a distance of 15.875 microns.

The 15.875 micron inter-dot distance coupled with mems requirements mean that the horizontal distance between two adjacent nozzles on a single row (e.g. firing even dots) will be 31.75 microns.

All 640 dots in an odd or even colour row are exactly aligned vertically. Rows are fired sequentially, so a complete row is fired in small fraction (nominally one tenth) of a line time, with individual nozzle firing distributed within this row time. As a result dots can end up on the paper with a vertical misplacement of up to one tenth of the dot pitch. This is considered acceptable.

The vertical distance between rows is adjusted based on the row firing order. Firing can start with any row, and then follows a fixed rotation. FIG. 78 shows the default row firing order from 1 to 10, starting at the top even row. Rows are separated by an exact number of dot lines, plus a fraction of a dot line corresponding to the distance the paper will move between row firing times. This allows exact dot-on-dot printing for each colour. The starting row can be varied to correct for vertical misalignment between chips, to the nearest 0.1 pixels. SoPEC appropriate delays each row's data to allow for the spacing and firing order

An additional constraint is that the odd and even rows for given colour must be placed close enough together to allow them to share an ink channel. This results in the vertical spacing shown in FIG. 79, where L represents one dot pitch.

Multiple identical printhead chips must be capable of being linked together to form an effectively horizontal assembled printhead.

Although there are several possible internal arrangements, construction and assembly tolerance issues have made an internal arrangement of a dropped triangle (ie a set of rows) of nozzles within a series of rows of nozzles, as shown in FIG. 80. These printheads can be linked together as shown in FIG. 80.

Compensation for the triangle is preferably performed in the printhead, but if the storage requirements are too large, the triangle compensation can occur in SoPEC. However, if the compensation is performed in SoPEC, it is required in the present embodiment that there be an even number of nozzles on each side of the triangle.

It will be appreciated that the triangle disposed adjacent one end of the chip provides the minimum on-printhead storage requirements. However, where storage requirements are less critical, other shapes can be used. For example, the dropped rows can take the form of a trapezoid.

The join between adjacent heads has a 45° angle to the upper and lower chip edges. The joining edge will not be straight, but will have a sawtooth or similar profile. The nominal spacing between tiles is 10 microns (measured perpendicular to the edge). SoPEC can be used to compensate for both horizontal and vertical misalignments of the print heads, at some cost to memory and/or print quality.

Note also that paper movement is fixed for this particular design.

A print rate of 60 A4/Letter pages per minute is possible. The printhead will assume the following:

-   -   page length=297 mm (A4 is longest page length)     -   an inter-page gap of 60 mm or less (current best estimate is         more like 15+/−5 mm

This implies a line rate of 22,500 lines per second. Note that if the page gap is not to be considered in page rate calculations, then a 20 KHz line rate is sufficient.

Assuming the page gap is required, the printhead must be capable of receiving the data for an entire line during the line time. i.e. 5 colors 1280 dots 22,500 lines=144 MHz or better (173 MHz for 6 colours).

It will be appreciated by those skilled in the art that the foregoing represents only a preferred embodiment of the present invention. Those skilled in the relevant field will immediately appreciate that the invention can be embodied in many other forms. 

1. A printhead controller for supplying print data to a printhead having first and second rows of nozzles, the nozzles of the first row being spaced from and in-line with a corresponding nozzle of the second row in a direction of print media propagation, the controller being configured to supply the print data to the printhead such that, in the event a nozzle in the first row is faulty, dot data corresponding to the faulty nozzle in the first row is sent to a corresponding nozzle in the second row, whereby the corresponding nozzle of the second row prints at a position on the print media at a position where the faulty nozzle would otherwise have printed.
 2. A printhead controller according to claim 1, wherein the printhead has a plurality of sets of the first and second rows.
 3. A printhead controller according to claim 2, wherein each of the sets of the first and second rows is configured to print in a single ink color.
 4. A printhead controller according to claim 1, wherein each of the first and second rows respectively includes an odd and an even sub-row, the odd and even sub-rows being spaced with respect to each other in a direction of print media.
 5. A printhead controller according to claim 4, wherein the odd and even sub-rows are transversely offset with respect to each other.
 6. A printer comprising: a printhead having first and second rows of nozzles, each nozzle of the first row being spaced from and in-line with a corresponding nozzle of the second row in a direction of print media propagation; and a printhead controller for supplying print data to the printhead, the printhead controller supplying dot data corresponding to a faulty nozzle in the first row to a nozzle in the second row corresponding to the faulty nozzle, whereby the nozzle in the second row corresponding to the faulty nozzle in the first row prints at a position on the print media where the faulty nozzle would otherwise have printed. 